Ion-conducting structures, devices including ion-conducting structures, and methods for use and fabrication thereof

ABSTRACT

An ion-conducting structure comprises a metal-fibril complex formed by one or more elementary nanofibrils. Each elementary nanofibril can be composed of a plurality of cellulose molecular chains with functional groups. Each elementary nanofibril can also have a plurality of metal ions. Each metal ion can act as a coordination center between the functional groups of adjacent cellulose molecular chains so as to form a respective ion transport channel between the cellulose molecular chains. The metal-fibril complex can comprise a plurality of second ions. Each second ion can be disposed within one of the ion transport channels so as to be intercalated between the corresponding cellulose molecular chains. In some embodiments, the metal-fibril complex is formed as a solid-state structure.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. ProvisionalApplication No. 62/890,404, filed Aug. 22, 2019, which is herebyincorporated by reference herein in its entirety.

FIELD

The present disclosure relates generally to ion-conducting structures,and more particularly, to ion-conducting structures formed of elementarynanofibrils having a plurality of polymer molecular chains, methods foruse and fabrication of such ion-conducting structures, and devicesincluding such ion-conducting structures.

SUMMARY

Embodiments of the disclosed subject matter provide ion-conductingstructures formed by one or more elementary nanofibrils whose polymermolecular chains have been chemically modified, as well as devicesincluding such ion-conducting structures, and methods for fabricationand use thereof. For example, the elementary nanofibril(s) can be anaturally-occurring polysaccharide, such as cellulose, chitin, orchitosan. In some embodiments, the chemical modification of theelementary nanofibril(s) involves breaking hydrogen bonds between thepolymer molecular chains to allow metal ions to form respectivecoordination bonds between exposed functional groups of adjacent polymermolecular chains. The coordination metal bonds thus space the polymerchains apart to form ion transport channels through the elementarynanofibril(s). The combination of elementary nanofibril(s) and metalions forms a metal-fibril complex. In some embodiments, second ions canbe intercalated between the polymer molecular chains. In someembodiments, the metal-fibril complex with intercalated second ions canbe dried to form a solid-state ion-conducting structure. In someembodiments, the chemical modification of the elementary nanofibril(s)can include modifying charge density or charge type of functional groupsof the polymer molecular chains. The resulting ion-conducting structurescan be used as a component in electrical energy storage devices,electrical power generations systems (e.g., electricity generatingdevices), ion regulation or separation devices, ion conductioncomponents, and/or biological applications.

In a representative embodiment, an ion-conducting structure comprises ametal-fibril complex formed by one or more elementary nanofibrils. Eachelementary nanofibril is composed of a plurality of cellulose molecularchains with functional groups. Each elementary nanofibril can have aplurality of metal ions. Each metal ion can act as a coordination centerbetween the functional groups of adjacent cellulose molecular chains soas to form a respective ion transport channel between the cellulosemolecular chains. The metal-fibril complex can also comprise a pluralityof second ions. Each second ion can be disposed within one of the iontransport channels so as to be intercalated between the correspondingcellulose molecular chains. The metal-fibril complex can be asolid-state structure.

In another representative embodiment, a battery comprises a firstelectrode, a second electrode, and a separator membrane. The separatormembrane can be between the first and second electrodes. The separatormembrane can comprise a solid-state metal-fibril complex. One of thefirst and second electrodes can operate as a cathode, and the other ofthe first and second electrodes can operate as an anode. The solid-statemetal-fibril complex can be formed by a plurality of first nanofibrils.Each first nanofibril can be composed of a plurality of cellulosemolecular chains with first functional groups. Each first nanofibril canhave a plurality of first metal ions. Each first metal ion can act as afirst coordination center between the first functional groups ofadjacent cellulose molecular chains so as to form a respective first iontransport channel through the separator membrane. The solid-statemetal-fibril complex can comprise a plurality of second ions. Eachsecond ion can be disposed within one of the first ion transportchannels so as to be intercalated between the corresponding cellulosemolecular chains.

In another representative embodiment, a battery comprises a firstelectrode, a second electrode, and a separator between the first andsecond electrodes. One of the first and second electrodes can operate asa cathode, and the other of the first and second electrodes can operateas an anode. The separator can comprise a solid-state electrolyte. Thefirst electrode, the second electrode, or both the first and secondelectrodes comprises a solid-state metal-fibril complex. The solid-statemetal-fibril complex can be formed by a plurality of nanofibrils. Eachnanofibril can be composed of a plurality of cellulose molecular chainswith functional groups. Each nanofibril can have a plurality of metalions. Each metal ion can act as a coordination center between thefunctional groups of adjacent cellulose molecular chains so as to form arespective ion transport channel between the cellulose molecular chains.The solid-state metal-fibril complex can comprise a plurality of secondions. Each second ion can be disposed one of the ion transport channelsso as to be intercalated between the corresponding cellulose molecularchains.

In another representative embodiment, a method can comprise (a) forminga metal-fibril complex by immersing a plurality of elementarynanofibrils within an alkaline solution having a concentration of atleast 5% (w/v) and a plurality of metal ions dissolved therein. Eachelementary nanofibril can be composed of a plurality of cellulosemolecular chains with functional groups.

The immersing of (a) can be such that hydrogen bonds between adjacentfunctional groups of the cellulose molecular chains are broken so as toexpose the functional groups and such that the dissolved metal ions fromthe alkaline solution form coordination bonds with the exposedfunctional groups. The method can further comprise (b), after (a),intercalating second ions between adjacent cellulose molecular chains ofthe metal-fibril complex by immersing the metal-fibril complex in afirst solution having a plurality of the second ions dissolved therein.The method can also comprise (c), after (a), replacing free water in themetal-fibril complex by immersing the metal-fibril complex in an organicsolvent. The method can further comprise (d), after (c), drying themetal-fibril complex such that a total content of water within themetal-fibril complex is less than or equal to 10 wt %, thereby formingthe metal-fibril complex with intercalated second ions as a solid-stateion conducting structure. In some embodiments, the first solution can bethe organic solvent, and the intercalating of (b) and the replacing freewater of (c) can be performed simultaneously. In other embodiments, thefirst solution can be separate from the organic solvent, and theintercalating of (b) can be performed before or after the replacing freewater of (c).

In another representative embodiment, an ion-conducting structurecomprises a metal-fibril complex. The metal-fibril complex can be formedby one or more elementary nanofibrils. Each elementary nanofibril can becomposed of a plurality of polymer molecular chains with functionalgroups. Each elementary nanofibril can have a plurality of metal ions.Each metal ion can act as a coordination center between the functionalgroups of adjacent molecular chains so as to form a respective iontransport channel between the molecular chains.

In another representative embodiment, a battery comprises a first andsecond electrodes, and a solid electrolyte membrane between the firstand second electrodes. One of the first and second electrodes canoperate as a cathode, and the other of the first and second electrodescan operate as an anode. The first electrode, the second electrode, thesolid electrolyte membrane, or any combination thereof can comprise anion-conducting structure, which comprises a metal-fibril complex. Themetal-fibril complex can be formed by one or more elementarynanofibrils. Each elementary nanofibril can be composed of a pluralityof polymer molecular chains with functional groups. Each elementarynanofibril can have a plurality of metal ions. Each metal ion can act asa coordination center between the functional groups of adjacentmolecular chains so as to form a respective ion transport channelbetween the molecular chains.

In another representative embodiment, a fuel cell comprises first andsecond electrodes, and a proton exchange membrane between the first andsecond electrodes. One of the first and second electrodes can operate asa cathode, and the other of the first and second electrodes can operateas an anode. The first electrode, the second electrode, the protonexchange membrane, or any combination thereof can comprise anion-conducting structure, which comprises a metal-fibril complex. Themetal-fibril complex can be formed by one or more elementarynanofibrils. Each elementary nanofibril can be composed of a pluralityof polymer molecular chains with functional groups. Each elementarynanofibril can have a plurality of metal ions. Each metal ion can act asa coordination center between the functional groups of adjacentmolecular chains so as to form a respective ion transport channelbetween the molecular chains.

In another representative embodiment, a method can comprise forming ametal-fibril complex by immersing a plurality of elementary nanofibrilswithin an alkaline solution and a plurality of metal ions dissolvedtherein. Each elementary nanofibril can be composed of a plurality ofpolymer molecular chains with functional groups. The immersing can besuch that hydrogen bonds between adjacent functional groups of thepolymer molecular chains are broken so as to expose the functionalgroups and such that the dissolved metal ions from the alkaline solutionform coordination bonds with the exposed functional groups.

In another representative embodiment, a method can comprise conductingions using one or more elementary nanofibrils. Each elementarynanofibril can be composed of a plurality of polymer molecular chainswith functional groups that have been chemically-modified. In someembodiments, the chemical-modification can include forming acoordination bond between a metal ion and functional groups of adjacentpolymer molecular chains of the nanofibril. In some embodiments, thechemical-modification can include converting hydroxyl groups of thepolymer molecular chains to carboxyl groups, for example, using a(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) treatment. In someembodiments, the chemical-modification can include etherification of thefunctional groups, for example, using a 3-chloro-2-hydroxypropyltrimethyl ammonium chloride (CHPTAC) treatment.

This summary is not intended to identify key features or essentialfeatures of the claimed subject matter, nor is it intended to be used tolimit the scope of the claimed subject matter. The foregoing and otherobjects, features, and advantages of the invention will become moreapparent from the following detailed description, which proceeds withreference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will hereinafter be described with reference to theaccompanying drawings, which have not necessarily been drawn to scale.Where applicable, some elements may be simplified or otherwise notillustrated in order to assist in the illustration and description ofunderlying features. Throughout the figures, like reference numeralsdenote like elements.

FIG. 1 is a simplified schematic diagram illustrating the hierarchicalaligned structure of cellulose fibers in natural wood.

FIG. 2A is a simplified schematic diagram illustrating adjacent polymermolecular chains in an exemplary elementary nanofibril in an originalunmodified state, according to one or more embodiments of the disclosedsubject matter.

FIG. 2B is a simplified schematic diagram illustrating the nanofibril ofFIG. 2A after immersion in an alkaline solution, thereby opening thespace between the polymer molecular chains, according to one or moreembodiments of the disclosed subject matter.

FIG. 2C is a simplified schematic diagram illustrating the nanofibril ofFIG. 2B after bonding of dissolved metal ions from the alkaline solutionto the functional groups of adjacent molecular chains, thereby forming ametal-fibril complex, according to one or more embodiments of thedisclosed subject matter.

FIG. 2D is a simplified schematic diagram illustrating the metal-fibrilcomplex of FIG. 2C after intercalation of second ions between thefunctional groups of adjacent molecular chains, according to one or moreembodiments of the disclosed subject matter.

FIG. 2E is a simplified schematic diagram illustrating the metal-fibrilcomplex of FIG. 2D after solvent exchange, according to one or moreembodiments of the disclosed subject matter.

FIG. 2F is a simplified schematic diagram illustrating the metal-fibrilcomplex of FIG. 2E after drying, thereby forming a solid-statestructure, according to one or more embodiments of the disclosed subjectmatter.

FIGS. 3A-3B are simplified process flow diagrams for exemplary methodsfor fabricating a solid-state ion-conducting structure, according to oneor more embodiments of the disclosed subject matter.

FIGS. 4A-4D are simplified perspective views of exemplary ion-conductingmetal-fibril complexes formed as substantially-planar structures,according to one or more embodiments of the disclosed subject matter.

FIGS. 4E-4F are simplified schematic diagrams illustratingcross-sectional views of another ion-conducting metal-fibril complexformed by a single elementary nanofibril, according to one or moreembodiments of the disclosed subject matter.

FIGS. 5A-5E are simplified schematic diagrams of exemplary batterysystems employing solid-state ion-conducting metal-fibril complexes forone or more components, according to one or more embodiments of thedisclosed subject matter.

FIG. 6A is a simplified schematic diagram of a fuel cell systememploying an ion-conducting metal-fibril complex, according to one ormore embodiments of the disclosed subject matter.

FIG. 6B is a simplified schematic diagram of a supercapacitor systememploying a solid-state ion-conducting metal-fibril complex, accordingto one or more embodiments of the disclosed subject matter.

FIG. 7 is a simplified schematic diagram illustrating an aqueousion-conducting structure formed by a metal-fibril complex withintercalated second ions, according to one or more embodiments of thedisclosed subject matter.

FIG. 8 a simplified process flow diagram for an exemplary method forfabricating an aqueous ion-conducting structure, according to one ormore embodiments of the disclosed subject matter.

FIG. 9 is a simplified schematic diagram of an exemplary battery systememploying an aqueous ion-conducting metal-fibril complex for one or morecomponents, according to one or more embodiments of the disclosedsubject matter.

FIGS. 10A-10B are simplified schematic views of cellulose nanofibrilswithin a wood structure before and after delignification, respectively,in forming an exemplary ion-conducting structure, according to one ormore embodiments of the disclosed subject matter.

FIG. 10C is a simplified schematic diagram illustrating adjacentcellulose molecular channels in an exemplary cellulose nanofibril afterchemical modification to act as an ion transport channel, according toone or more embodiments of the disclosed subject matter.

FIG. 11 is a simplified process flow diagram for a method forfabricating an ion-conducting structure from wood, according to one ormore embodiments of the disclosed subject matter.

FIG. 12 is a simplified schematic diagram of an exemplary thermoelectricsystem employing a wood-based ion-conducting structure, according to oneor more embodiments of the disclosed subject matter.

FIG. 13 is a simplified schematic diagram of an exemplary transistoremploying a wood-based ion-conducting structure, according to one ormore embodiments of the disclosed subject matter.

FIG. 14 is a simplified schematic diagram of an exemplary osmotic powergeneration system employing a wood-based ion selective structure,according to one or more embodiments of the disclosed subject matter.

FIG. 15A is a simplified perspective view of a natural woodmicrostructure used in forming an exemplary ion-conducting structure,according to one or more embodiments of the disclosed subject matter.

FIG. 15B shows a simplified perspective view (left) of an exemplary woodmicrostructure after chemical treatment and densification, and asimplified schematic diagram (right) illustrating adjacent cellulosemolecular chains of the exemplary wood microstructure, according to oneor more embodiments of the disclosed subject matter.

FIG. 16 is a simplified process flow diagram for another exemplarymethod for fabricating an ion-conducting structure from wood, accordingto one or more embodiments of the disclosed subject matter.

FIG. 17 is a graph of electrochemical impedance spectra (EIS) for afabricated example of a solid-state metal-fibril complex (e.g., Cu-paperwith intercalated Li ions) at different temperatures.

FIGS. 18A-18B are graphs of EIS at different conductor lengths andresistance versus conductor length, respectively, for a fabricatedexample of a solid-state metal-fibril complex (e.g., Cu-wood withintercalated Li ions).

FIG. 18C is a graph comparing thermal gravimetric analysis (TGA) curvesfor delignified wood without any metal (e.g., white wood example), anexample of Cu-wood treated with dimethylformamide (DMF) replacement, andan example of Cu-wood treated with electrolyte.

FIG. 18D is a graph comparing tensile stress-strain curves for the whitewood example treated with DMF and electrolyte, and for the example ofCu-wood treated with electrolyte.

FIG. 18E is a graph of EIS for another fabricated example of asolid-state metal-fibril complex (e.g., Cu-wood with intercalated Naions).

FIG. 19 is a graph comparing stress-stain curves for crystallinecellulose (e.g., cellulose II) and a fabricated example of ametal-fibril complex (e.g., Cu-cellulose II).

FIG. 20A is a graph of conductivities versus NaOH solution concentrationfor bulk solution and a fabricated example of a metal-fibril complex(e.g., Cu-cellulose) within the solution.

FIG. 20B is a graph of conductivities versus KOH solution concentrationfor bulk solution and the fabricated example of a metal-fibril complex(e.g., Cu-cellulose) within the solution.

FIG. 20C is a graph of mobility enhancement offered by the fabricatedexample of a metal-fibril complex (e.g., Cu-cellulose) with respect toNa+ and K+ ions.

FIG. 21 is a graph of measured current-voltage characteristics of NaClin bulk solution and a fabricated example of a battery employing anaqueous metal-fibril complex (e.g., Cu-cellulose filled with dilutedNaCl in solution).

FIG. 22A is a graph comparing charge densities for natural wood, acellulosic membrane (e.g., delignified wood) without chemicalmodification, and a fabricated example of a cellulosic membrane that hasbeen chemical modified (e.g., TEMPO-oxidized).

FIG. 22B is a graph of conductance of the cellulosic membrane withoutchemical modification as a function of electrolyte (e.g., NaOH)concentration.

FIG. 22C is a graph comparing measured differential thermal voltages forbulk electrolyte (e.g., NaOH), a polymer electrolyte (e.g.,NaOH+poly(ethylene oxide) (PEO)+deionized water), and variouscellulose-based structures.

FIG. 23A is a graph comparing thermal charging behavior of the polymerelectrolyte to the chemically-modified cellulosic membrane infiltratedwith the polymer electrolyte.

FIG. 23B is a graph comparing discharging behavior of the polymerelectrolyte to the chemically-modified cellulosic membrane infiltratedwith the polymer electrolyte.

FIG. 24A is a graph of the zeta-potential of cellulose fibers andoxidized/surface-charged cellulose under neutral pH for a celluloseconcentration of ˜0.1%.

FIG. 24B are simplified schematic diagrams illustrating cellulosemolecular chains before (top) and after (bottom) TEMPO modification.

FIG. 24C is a graph of conductivity versus KCl concentration for afabricated example of a cellulose membrane in KCl solution before andafter chemical modification.

FIG. 24D is a graph of conductivity versus KCl concentration for afabricated example of a cellulose membrane in KCl solution before andafter densification.

FIG. 25A is a graph of current versus voltage of a fabricated example ofa cellulose-based ionic transistor for different applied gate voltages.

FIG. 25B is a graph of current versus gate voltage of the fabricatedexample of the cellulose-based ionic transistor, with the inset showingthe same results on a semi-log scale.

FIG. 26A is a graph of current versus voltage of a fabricated example ofa polymer-filled, delignified, chemically-modified wood membrane in anosmotic power generation setup.

FIG. 26B is a graph of resulting output power resulting from iontransport through the fabricated example of the polymer-filled,delignified, chemically-modified wood membrane in the osmotic powergeneration setup.

FIG. 27A is a graph of electrochemical impedance spectra (EIS) atdifferent KCl concentrations for a fabricated example of achemically-modified, densified wood membrane.

FIG. 27B is a graph comparing ionic conductivity versus KClconcentration for KCl bulk solution and various fabricated examples ofwood-based structures.

FIG. 27C is a graph comparing the ion conductivity values of FIG. 27B ata concentration of 10⁻⁴M KCl.

FIG. 28A is a graph comparing current versus voltage characteristics ofnatural wood and a fabricated example of a cationic wood membrane.

FIG. 28B is a graph of energy dispersive x-ray spectroscopy (EDS)elemental spectrum of chemically-modified wood used in the fabricatedexample of the cationic wood membrane.

FIGS. 29A-29B are simplified schematic diagrams illustrating a chitosanmolecular chain prior to metal coordination and adjacent chitosanmolecular chains after metal coordination, according to one or moreembodiments of the disclosed subject matter.

FIG. 29C is a graph of proton conductivities as a function of methanolpermeability for various fuel cell separator membranes, including afabricated example of a metal-fibril complex (e.g., Cu-chitosan).

DETAILED DESCRIPTION General Considerations

For purposes of this description, certain aspects, advantages, and novelfeatures of the embodiments of this disclosure are described herein. Thedisclosed methods, structures, and devices should not be construed asbeing limiting in any way. Instead, the present disclosure is directedtoward all novel and nonobvious features and aspects of the variousdisclosed embodiments, alone and in various combinations andsub-combinations with one another. The methods, structures, and devicesare not limited to any specific aspect or feature or combinationthereof, nor do the disclosed embodiments require that any one or morespecific advantages be present, or problems be solved. The technologiesfrom any embodiment or example can be combined with the technologiesdescribed in any one or more of the other embodiments or examples. Inview of the many possible embodiments to which the principles of thedisclosed technology may be applied, it should be recognized that theillustrated embodiments are exemplary only and should not be taken aslimiting the scope of the disclosed technology.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed methods can be used in conjunction with other methods.Additionally, the description sometimes uses terms like “provide” or“achieve” to describe the disclosed methods. These terms are high-levelabstractions of the actual operations that are performed. The actualoperations that correspond to these terms may vary depending on theparticular implementation and are readily discernible by one of ordinaryskill in the art.

As used herein, “comprising” means “including” and the singular forms“a” or “an” or “the” include plural references unless the contextclearly dictates otherwise. The term “or” refers to a single element ofstated alternative elements or a combination of two or more elements,unless the context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms usedherein have the same meaning as commonly understood to one of ordinaryskill in the art to which this disclosure belongs. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present disclosure, suitable methods andmaterials are described below. The materials, methods, and examples areillustrative only and not intended to be limiting. Other features of thedisclosure are apparent from the following detailed description and theclaims.

The disclosure of numerical ranges should be understood as referring toeach discrete point within the range, inclusive of endpoints, unlessotherwise noted. Unless otherwise indicated, all numbers expressingquantities of components, molecular weights, percentages, temperatures,times, and so forth, as used in the specification or claims are to beunderstood as being modified by the term “about.” Accordingly, unlessotherwise implicitly or explicitly indicated, or unless the context isproperly understood by a person of ordinary skill in the art to have amore definitive construction, the numerical parameters set forth areapproximations that may depend on the desired properties sought and/orlimits of detection under standard test conditions/methods, as known tothose of ordinary skill in the art. When directly and explicitlydistinguishing embodiments from discussed prior art, the embodimentnumbers are not approximates unless the word “about” is recited.Whenever “substantially,” “approximately,” “about,” or similar languageis explicitly used in combination with a specific value, variations upto and including 10% of that value are intended, unless explicitlystated otherwise.

Directions and other relative references may be used to facilitatediscussion of the drawings and principles herein, but are not intendedto be limiting. For example, certain terms may be used such as “inside,”“outside,”, “top,” “bottom,” “interior,” “exterior,” “left,” right,”“front,” “back,” “rear,” and the like. Such terms are used, whereapplicable, to provide some clarity of description when dealing withrelative relationships, particularly with respect to the illustratedembodiments. Such terms are not, however, intended to imply absoluterelationships, positions, and/or orientations. For example, with respectto an object, an “upper” part can become a “lower” part simply byturning the object over. Nevertheless, it is still the same part and theobject remains the same.

Overview of Terms

The following explanations of specific terms and abbreviations areprovided to facilitate the description of various aspects of thedisclosed subject matter and to guide those of ordinary skill in the artin the practice of the disclosed subject matter.

Elementary nanofibril: A basic nanoscale, elongated structure comprisedof a plurality of polymer molecular chains (e.g., 10-36 chains) stackedin parallel or antiparallel directions. For example, nanofibrils canhave an original (e.g., unmodified) diameter of 5 nm or less.

Microfibril (also referred to as nanofibril aggregate): A microscale,elongated structure comprised of a plurality of elementary nanofibrilsarranged in parallel. For example, microfibrils can have an original(e.g., unmodified) diameter on the order of 1 μm-10 μm.

Fiber (also referred to as macrofibril): An elongated structurecomprised of a plurality of microfibrils arranged in parallel. Forexample, fibers can have an original (e.g., unmodified) diameter on theorder of 100 μm-1 mm.

Functional group: A group of atoms or molecules of the polymer molecularchain that can be exposed or modified to provide the disclosed iontransport properties between polymer molecular chains. In someembodiments, the functional groups exposed are NH₂ molecules, OHmolecules, and/or O atoms.

Metal fibril complex: A structure formed by one or more elementarynanofibrils, with metal ions coordinate-bonded between functional groupsof adjacent polymer molecular chains within the nanofibrils.

Coordinate bond: A covalent dipolar bond between a metal donor ion andsurrounding ligands (e.g., the functional groups of polymer molecularchannels), with the metal ion acting as a coordination center.

Solid-state: A substantially-solid structure with substantially nomotive or flowable liquid (also referred free liquid) therein. Inembodiments, total liquid within the structure is less than 10%, andpreferably the amount of bound liquid therein is less than 8%.

Free liquid (e.g., free water): Liquid within a structure that is not inchemical combination with the structure, such that the liquid is capableof moving within or through the structure.

Bound liquid (e.g., bound water): Liquid within a structure that is inchemical combination with the structure, such that liquid cannot movewithin or through the structure.

INTRODUCTION

Natural wood has a unique three-dimensional porous structure withmultiple channels, including vessels and tracheid lumina (e.g., tubularchannels of 20-80 μm in cross-sectional dimension) extending in adirection of wood growth. Walls of cells in the natural wood areprimarily composed of cellulose (40 wt %˜50 wt %), hemicellulose (20 wt%˜30 wt %), and lignin (20 wt %˜35 wt %), with the three componentsintertwining with each other to form a strong and rigid wall structure.Cellulose fibers in the secondary cell wall (S2 layer) of the naturalwood are substantially aligned along the wood growth direction. Thenaturally-occurring cellulose exhibits a hierarchical structure, whichcan be exploited in embodiments to provide unique or improved iontransport properties. For example, as shown in FIG. 1, the natural woodcell 100 has a plurality of cellulose fibers 102 surrounding andextending substantially parallel to lumen 104. The cellulose fibers 102can be separated into constituent high-aspect-ratio microfibrils 106 inthe form of aggregated three-dimensional networks (e.g., as bundles)that provide relatively high surface area. The cellulose microfibrils106 can be further subdivided into elementary nanofibrils 108, which arecomposed of 12-36 linear polymer molecular chains 110. Each polymermolecular chain 110 is formed of thousands of repeating glucose unitsconnected by strong covalent bonds that are arranged in a highly-orderedcrystalline structure. The polymer molecular chains 110 are heldtogether in a densely-packed arrangement forming the elementarynanofibril 108 by intramolecular hydrogen bonding 112 between functionalgroups 114 of adjacent molecular chains 110.

In embodiments of the disclosed subject matter, polymer molecular chainsof the elementary nanofibril can be chemically-modified to alter orimprove ion transport properties thereof. In some embodiments, chargedensity and/or type (e.g., negative or positive) of the functionalgroups can be modified by appropriate chemical treatment. Cellulosenaturally exhibits a negative surface charge due to dissociation of itshydroxyl functional groups. For example, in some embodiments, a(2,2,6,6-Tetramethylpiperidyl-1-Oxyl) oxidation (TEMPO) treatment can beemployed to convert the hydroxyl functional groups to carboxyl groups.Since the carboxyl groups dissociate more readily in solution (e.g.,water), a higher surface charge density and zeta potential can beachieved as compared to native cellulose. In other embodiments, forexample, 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) canbe employed as an etherifying agent to modulate the surface charge ofthe cellulose (e.g., to present a positive charge in solution).

In some embodiments, the chemical modification can include forming theelementary nanofibril into a metal-fibril complex. Hydrogen bondsbetween functional groups of the cellulose molecular chains can bebroken by immersing the elementary nanofibril in an alkaline solution,thereby increasing spacing between adjacent cellulose molecular chainsand exposing the functional groups. Metal ions dissolved in the alkalinesolution can diffuse into the enlarged space between the adjacentcellulose molecular chains and can form coordination bonds between theexposed functional groups, thereby forming a metal-fibril complex (e.g.,metal-cellulose). Such metal can include, for example, copper (Cu), zinc(Zn), aluminum (Al), calcium (Ca), iron (Fe), or any combinationthereof. In some embodiments, respective ion transport channels areformed by the enlarged spaces between cellulose molecular chains in thenanofibril, and the spaced arrangement of the cellulose molecular chainsis maintained by the metal coordinate bonds.

In some embodiments, the chemical modification can include intercalatingsecond ions within the metal-fibril complex to form an ion-conductingstructure. For example, second ions from solution can diffuse into theion transport channels of the metal-fibril complex. Such second ions caninclude, for example, lithium (Li+), sodium (Na+), potassium (K+),magnesium (Mg+), proton (H+), or any combination thereof. Alternativelyor additionally, in some embodiments, the second ions can include aproton donor molecule, such as ammonium (NH₄+).

In some embodiments, the metal-fibril complex can be further processedinto a solid-state structure with second ions retained therein. Forexample, in some embodiments, the metal-fibril complex with second ionsintercalated therein is removed from solution and dried to form thesolid-state structure. In some embodiments, prior to drying, a solventexchange can be performed to replace water in the metal-fibril complexwith an organic solvent (e.g., a polar aprotic solvent), and organicsolvent is evaporated during the drying. For example, the organicsolvents can include, but are not limited to, dimethylformamide (DMF),dimethyl sulfoxide (DMSO), ethylene glycol diglycidyl ether (EGDGE),propylene carbonate (PC), and acetone. In some embodiments, selection ofthe organic solvent for the solvent exchange/drying process can adaptthe crystal structure of the cellulose in the final metal-fibril complexfor a particular application. For example, when the solvent is DMSOand/or EGDGE, the cellulose in the metal-fibril complex can retain theircrystalline morphology. Alternatively, when the solvent is DMF, PC,and/or acetone, the cellulose in the metal-fibril complex can have anamorphous morphology.

Alternatively, in some embodiments, the metal-fibril complex can bedried without second ions intercalated therein to form a preliminarystructure, and the second ions can be introduced to the preliminarystructure at a later time. In some embodiments, the metal-fibril complexwith second ions intercalated therein can be formed as an initialsolid-structure even though its subsequent usage in a particularapplication may otherwise increase the content of free liquid therein,for example, when the metal-fibril complex is used as a proton exchangemembrane of a fuel cell.

In some embodiments, the cellulose elementary nanofibrils can be sourcedfrom natural wood (e.g., trees), as discussed above. The natural woodcan be any type of hard wood or softwood, such as, but not limited to,basswood, oak, poplar, ash, alder, aspen, balsa wood, beech, birch,cherry, butternut, chestnut, cocobolo, elm, hickory, maple, oak, padauk,plum, walnut, willow, yellow poplar, bald cypress, cedar, cypress,douglas fir, fir, hemlock, larch, pine, redwood, spruce, tamarack,juniper and yew. Alternatively or additionally, in some embodiments, thecellulose elementary nanofibrils can be sourced from other fibrous plantsources (e.g., bamboo, grass, cotton, ramie fiber, etc.), bacteriasources, and/or any other fibrous cellulose source.

In some embodiments, the processing of the elementary cellulosenanofibrils from an initial source can employ a “top-down” approach totake advantage of an existing microstructure arrangement of the sourcematerial. For example, the elementary cellulose nanofibrils within apiece of natural wood can be subjected to one or more of the chemicalmodifications described herein. The piece of natural wood can be cut inany direction with respect to its growth direction. Since the cellulosefibers are naturally aligned with the growth direction, the direction ofthe cut will dictate the orientation of the elementary nanofibrils inthe final structure, which orientation can imbue the resulting structurewith unique ion transport and/or mechanical properties. For example, insome embodiments, the piece of natural wood can be vertical cut (e.g.,parallel to tree growth direction) such that resulting cellulose fibersare oriented substantially parallel to a major face (e.g., largestsurface area) of the cut wood piece. In some embodiments, the piece ofnatural wood can be horizontal or rotation cut (e.g., perpendicular totree growth direction), such that resulting fibers are orientedsubstantially perpendicular to the major face of the cut structure. Insome embodiments, the piece of natural wood can be cut at anyorientation between the horizontal and vertical cuts.

In some embodiments, the piece of natural wood can be subject todelignification prior to the desired chemical modification of theelementary cellulose nanofibrils. As used herein “delignified” or“delignification” refers to removing substantially all of the ligninfrom the natural wood, and “removing substantially all of the lignin”means that at least 90% of the lignin that naturally exits in the woodhas been removed. For example, the weight percentage (wt %) of ligninmay be reduced from over 20 wt % (e.g., 23.4 wt %) in natural wood toless than 5 wt % in the delignified wood, and preferably less than 1 wt% (e.g., ≤0.6 wt %). Concurrent with the lignin removal, some, most orsubstantially all of the hemicellulose may also be removed. In someembodiments, all of the lignin and hemicellulose can be removed, therebyproviding a cellulose-only structure. Exemplary processes for performingsuch delignification are described in, for example, InternationalPublication No. WO 2018/191181, published Oct. 18, 2018, InternationalPublication No. WO 2018/187238, published Oct. 11, 2018, andInternational Publication No. WO 2019/055789, published Mar. 21, 2019,which are publications are incorporated by reference herein.

In some embodiments, the piece of wood (whether natural or delignified)can be subject to densification prior to or after the desired chemicalmodification of the elementary cellulose nanofibrils. As used herein,“densification” refers to the process of pressing the wood in adirection crossing a direction of extension of the lumina (or a woodgrowth direction) of the wood, such that the lumina mostly or fullycollapse (e.g., such that the thickness of the wood is reduced by atleast 75%, for example, ˜90%). Exemplary processes for performing suchdensification are described in, for example, International PublicationNo. WO 2018/191181 and International Publication No. WO 2019/055789,which were incorporated by reference above.

In some embodiments, the “top-down” approach employs an initial sourcematerial having a patterned arrangement of cellulose fibers. Forexample, the initial source material can be woven fabric or textile(e.g., formed of cotton fibers). Applying the desired chemicalmodification to the initial source material can thus result in anion-conducting cellulose-based structure that inherits the patternedarrangement. In other embodiments, the “top-down” approach employs aninitial source material having a random arrangement of cellulose fibers.For example, the initial source material can be a piece of paper withrandom orientation of cellulose fibers. Applying the desired chemicalmodification to the initial paper can thus result in an ion-conductingcellulose-based structure that inherits the random arrangement.

In some embodiments, the processing of the elementary cellulosenanofibrils from an initial source can employ a “bottom-up” approach toprovide a final microstructure independent of the microstructure of thesource material. For example, a piece of natural wood (or other startingcellulose material) can be fibrillated prior to or after the desiredchemical modification. As used herein, “fibrillation” refers to theprocess of releasing the cellulose microfibrils and/or the elementarynanofibrils from the aggregate hierarchical structure (e.g., the parentstructure of natural wood). Fibrillation can be performed by any methodknown in the art, such as chemical means (e.g., chemical fibrillation,such as a TEMPO treatment), mechanical means (e.g., mechanicalfibrillation, such as disk grinding), and/or enzymatic means (e.g., anenzymatic fibrillation process employing canonical cellulase enzymes,such as endoglucanases, in combination with amorphogenesis-inducingproteins, such as lytic polysaccharide monooxygenases (LPMO), swolleninand hemicelluloses).

As compared to cellulose fibers (e.g., having a diameter of 100 μm to 1mm), the cellulose nanofibrils exposed by the fibrillation process havemuch larger aspect ratios (e.g., length-to-diameter of 200:1 to 1000:1)due to the much smaller diameter of the nanofibrils (e.g., diameter ≤5nm). Due to this large aspect ratio, in some embodiments, thenanofibrils can be used as an ion-conducting additive in solidelectrodes with a low ion-conductive percolation threshold (e.g., a lowminimum proportion of cellulose for ion conduction in the electrode).Moreover, in some embodiments, the cellulose nanofibrils exposed by thefibrillation process can be used to form relatively thin membranes(e.g., 50 μm to 200 μm) that are also sufficiently dense (e.g., no orminimal micro-scale pores). In some embodiments, these thin, densemembranes formed of the cellulose nanofibrils can be used as asolid-state electrolyte, for example, in a solid-state energy-storagesystem. In contrast, membranes formed of cellulose fibers (e.g.,conventional filter paper) are relatively thick (e.g., >300 μm) and maycontain large pores.

In some embodiments, the separated microfibrils and/or nanofibrils afterfibrillation can be reassembled into a new structure. For example, aslurry containing the separated microfibrils and/or nanofibrils can bevacuum-filtered and pressed to form a paper with random orientation ofmicrofibrils and/or nanofibrils. Alternatively or additionally, in someembodiments, the microfibrils and/or nanofibrils after fibrillation andchemical modification can be added to or incorporated with anothermaterial to form a final composite structure. For example, thefibrillated microfibrils and/or nanofibrils that have been chemicallymodified can combined with another material to form a conductiveelectrode. In some embodiments, the “top-down” approach reconfigures theinitial structure of a natural wood starting material, for example, as awire or cable. For example, a piece of natural wood can be directlyconverted into microfibers with diameters between 1 μm and 30 mm bypartial delignification (e.g., less than all lignin removed) followed bytwisting.

Although the above discussion and the following description ofembodiments focuses on cellulose for the elementary nanofibrils,embodiments of the disclosed subject matter are not limited thereto.Indeed, other naturally-occurring polysaccharides can also be used inplace of or in addition to cellulose, according to one or morecontemplated embodiments. For example, microfibrils can be formed ofcellulose, chitin, chitosan, or any combination thereof. Chitin is astructural polysaccharide made from chains of modified glucose and isfound in the exoskeletons of insects, the cell walls of fungi, andcertain hard structures in invertebrates and fish. Chitosan is a linearpolysaccharide made of glucosamine and N-acetyl glucosamine units.Chitosan can be formed by treating the chitin shells of shrimp and othercrustaceans with an alkaline substance (e.g., NaOH).

Alternatively or additional, in some embodiments, the elementarynanofibrils can be formed of any polymer molecular chains (for example,polymer molecular chains having polar functional groups (e.g., hydroxyl,carboxyl)). Such polymers molecular can include, but are not limited to,other polysaccharides (e.g., starch, pectin), poly(vinyl chloride)(PVC), poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA),poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(ethylmethacrylate) (PEMA), poly(methyl methacrylate) (PMMA), poly(ethyleneterephthalate) (PET), polyethylene (PE), poly(ethylene naphthalate)(PEN), polyamide (PA), poly(vinylidene chloride) (PVDC), and polylacticacid (PLA).

In some embodiments, the ion-conducting structure can be a compositeformed by a combination of the chemically-modified elementarynanofibrils and one or more additional materials. Such additionalmaterials can be infiltrated within the structure (e.g., in spacesbetween microfibrils, nanofibrils, and/or polymer molecular chains),bonded to the structure (e.g., to or between functional groups of thepolymer molecular chains), or added to the structure (e.g., on orcontacting the microfibrils and/or nanofibrils). For example, one ormore polymers can be mixed with the chemically-modified elementarynanofibrils. Such polymers can include, but are not limited to,polysaccharide (e.g., cellulose, chitin, chitosan, starch, pectin), PVC,PVA, PAA, PEO, PAN, PEMA, PMMA, PET, PE, PEN, PA, PVDC, and PLA.Alternatively or additionally, in some embodiments, the additionalmaterials can be a hydrogel or hydrogel precursor, and theion-conducting structure can be a hydrogel composite.

In some embodiments, for example, materials for infiltration within theion-conducting structure can include, but are not limited to, polymers,boron nitride (BN), carbon nanotubes (CNT), graphene, molybdenumdisulfide (MoS₂), and/or, metals. Infiltration of a polymer can beperformed using any method, such as, but not limited to, vacuuming,solvent exchange, and heating. After infiltration, a percentage ofpolymer within the ion-conducting structure can range from 1 wt % to 95wt %. For example, polymers used for impregnation can include, but arenot limited to, polysaccharide (e.g., chitin, chitosan, chitin starch,pectin), protein (e.g., osteogenic growth peptide, soy protein isolate,wheat protein, fibroin, spidroin, collagen, whey protein), plant oil(e.g., tung oil, catalpa oil, linseed oil, stearic acid, palmitic acid,oleic acid). Alternatively or additionally, polymer used forimpregnation can include, for example, synthetic macromolecules, suchas, but not limited to, PET, PP, PE, polystyrene (PS), PVC, PEN, PA,PVDC, and polylactic acid PLA. Exemplary processes for performing suchinfiltration and materials therefor are further described in, forexample, U.S. Pat. No. 10,411,222, issued Sep. 10, 2019, which isincorporated by reference herein.

In some embodiments, the resulting structure including thechemically-modified elementary nanofibrils can be employed in one ormore components of a device or system, such as, but not limited to,electrical energy storage devices (e.g., battery, supercapacitor, etc.),electrical power generation systems (e.g., fuel cell, thermoelectricpower generation device, osmotic power generation device), ionregulation or separation devices (e.g., cationic separation membrane,transistor), ion conduction components (e.g., nanofluidic ion conductor,ion-conducting additive, solid-state electrolyte), and biologicalapplications (e.g., ion regulation). In some embodiments, the structureincluding the chemically-modified elementary nanofibrils is formed as athin planar structure, for example, a membrane or sheet having athickness of 10 μm to 1000 μm, preferably 100 μm or less. Alternativelyor additionally, in some embodiments, the structure including thechemically-modified elementary nanofibrils reflects all or some of thethree-dimensional microstructure of the underlying source material,e.g., the aligned microstructure of the original natural wood or thewoven fiber pattern of the original textile.

Solid-State Metal-Fibril Complexes

As discussed above, in some embodiments, the polymer molecular chains ofone or more elementary nanofibrils can be chemically modified and driedto form a solid-state metal-fibril complex. For example, FIG. 2Aillustrates adjacent polymer molecular chains 210 of an exemplaryelementary nanofibril(s) in its original, unmodified state (e.g., nativestate). The polymer molecular chains 210 are held together in adensely-packed arrangement by hydrogen bonding 212 between functionalgroups of adjacent molecular chains 210. For example, the hydrogen bonds212 can maintain a spacing, W₁, between the polymer molecular chains 210of ≤1 nm (e.g., ≤about 0.6 nm for cellulose molecular chains). Byimmersing 216 the elementary nanofibril(s) in an alkaline solution(e.g., NaOH, KOH, LiOH), the hydrogen bonds 212 between functionalgroups can be broken, thereby allow the space between adjacent polymermolecular chains 210 to increase, as shown in FIG. 2B. For example, theterminal OH— groups of molecular chains within cellulose nanofibril(s)are exposed when immersed in the alkaline solution, due to the lowdissociation energy of the hydroxyl groups in the high-concentration(e.g., at least 5%) alkaline environment.

With the increased spacing between molecular chains 210, the elementarynanofibril(s) can be subjected to a metal ion treatment 220, where metalions 222 dissolved in solution can diffuse between the polymer molecularchains 210 and bond thereto. In particular, the dissolved metal ions 222can form a coordination bond with the exposed functional groups ofadjacent polymer molecular chains 210, as shown in FIG. 2C. In someembodiments, the dissolved metal ions are provided in thehigh-concentration alkaline solution used to open up the polymermolecular chains 210 in FIG. 2B, such that the immersing 216 and themetal ion treatment 220 occur simultaneously. Alternatively, in someembodiments, the metal ion treatment 220 is subsequent to thealkaline-solution immersing 216, for example, by dissolving a metal inthe alkaline solution after the elementary nanofibril(s) have beenimmersed therein, or by immersing the elementary nanofibril(s) in adifferent solution containing the dissolved metal ions.

For example, the metal ions 222 can maintain a spacing, W₂, between thepolymer molecular chains 210 that is greater than the native spacing,W₁. The metal 222 can be any metal capable of forming a coordinationbond with the functional groups of the polymer molecular chains 210, forexample, Cu, Zn, Al, Ca, and/or, Fe. For example, when the polymermolecular chains 210 are formed of cellulose and the metal ions includeCu, the Cu ions coordinate with the cellulose molecular chains byforming Cu(OH)₆ ⁴⁻ at O2, O3 sites of cellulose anhydrous glucose units(AGUs). The Cu ion can be coordinated with both O2 and O3 oxygens fromtwo neighboring chains, such that the overall Cu-cellulose complex formsa 3-dimensional cross-linked metal-organic-framework (MOF). The MOF canhave pores along the chain 210 direction that act as one-dimensional iontransport channel. For example, the diameter of the ion channel can beestimated as the distance between the loosely-bonded oxygen atoms as0.92 nm. In addition, the Cu-cellulose complex can have ˜0.5 nm wideopenings between neighboring ion transport channels, through which ionsand water molecules can hop between channel. In directions perpendicularto direction of extension of the polymer molecular chains 210, the 0.5nm wide openings require that ions shed solvation shell water moleculesin order to cross between channels, thereby leading to ion transportwith high mobility.

A metal-fibril complex is thus formed by coordination bonding of themetal ions 222 with functional groups of the adjacent polymer molecularchains 210 of the elementary nano-fibril. The metal-fibril complex canthen be subjected to a second ion treatment 224, where second ions 226are intercalated between the polymer molecular chains, as shown in FIG.2D. For example, the second ions can be Li+, Na+, K+, Mg+, and/or proton(H+). Alternatively or additionally, the second ions can include amolecule that donates a proton, such as ammonium ion (e.g., NH₄+). Insome embodiments, the second ions 226 are provided in thehigh-concentration alkaline solution (e.g., NaOH, KOH, LiOH, etc.) usedto open up the polymer molecular chains 210 in FIG. 2B, such that theimmersing 216 (and potentially metal ion treatment 220) occurssimultaneous with the second ion treatment 224.

Alternatively, in some embodiments, the second ion treatment 224 issubsequent to the metal ion treatment 220, for example, by dissolvingthe second ions in the alkaline solution after the dissolving the metaltherein, or by immersing the metal-fibril complex in a differentsolution containing the second ions 226. Thus, in some embodiments, thesecond ion treatment can include a solution containing an electrolytefor the desired second ions 226. For example, the solution for secondion treatment 224 can include propylene carbonate (PC), ethylenecarbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC),and/or diethyl carbonate (DEC). When the desired second ion comprisesLi, the Li-ion electrolyte can be, for example, LiClO₄, LiPF₆, LiBF₄,lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithiumbis(fluorosulfonyl)imide (LiFSI), or other similar salts. When thedesired second ion comprises protons, the added electrolyte can be, forexample, ammonia, ammonium nitrate, ammonium chloride, ammonium sulfate,polyacrylic acid, or citric acid. Beyond those specific examples listedherein, other electrolytes known in the art for the desired second ions226 could also be used.

The metal-fibril complex with intercalated second ions can then besubjected to solvent exchange treatment 228, where free water molecules218 within the elementary nanofibril(s) are replaced by organic solventmolecules 230, as shown in FIG. 2E. In some embodiments, the organicsolvent of treatment 228 can be a polar aprotic solvent. For example,the organic solvent can include DMF, DMSO, PC, acetone, and/or EGDGE.The solvent exchange treatment 228 can include immersing themetal-fibril complex in the organic solvent or washing the metal-fibrilcomplex with the organic solvent. In some embodiments, the immersing orwashing may be repeated multiple times (e.g., at least three times) toensure all free water 218 in the metal-fibril complex is removed. Insome embodiments, the second ions 226 are provided in the organicsolvent, such that the second ion treatment 224 occurs simultaneous withthe solvent exchange treatment 228. Alternatively, in some embodiments,the solvent exchange treatment 228 is before or after the second iontreatment 224.

Having been subjected to solvent exchange treatment 228, themetal-fibril complex can then be subjected to a drying treatment 232,where the organic solvent molecules 230 are evaporated while preservingthe nanostructure arrangement of the polymer molecular chains 210 andmetal ions 222 to form ion transport channels 234 (e.g., having a widthW2 of ˜1 nm and a spacing W3 of ≤˜2 nm) and second ions 226 intercalatedbetween the polymer molecular chains 210. In particular, after thedrying treatment 232, the metal-fibril complex is formed as asolid-state structure with minimal to no free liquid therein (althoughthere may otherwise be liquid molecules bound to the polymer molecularchains or other materials within the metal-fibril complex). For example,the solvent exchange 228 and drying 232 treatments can be such that thetotal liquid (e.g., water) within the metal-fibril complex is less than10 wt %, and preferably that the amount of bound liquid (e.g., water)within the metal-fibril complex is less than 8 wt %. For example, thedrying treatment 232 can include vacuum drying, freeze drying, and/orcritical point drying.

With native elementary nanofibrils, the electrostatic field adjacent tothe charged walls of the nanofibrils act to redistribute ions while themobility stays constant. The electrical double layer that regulates ionmovement is thus intrinsically limited to low electrolyte concentrationand cannot exceed the value for the bulk electrolyte under higherconcentrations. However, with the metal-fibril complex, sub-nanometerchannels 234 can be formed and tuned at the molecular scale, such thatconfinement of solvated ions 226 can be reduced to less than 1 nm. Newtransport phenomenon occurs within these sub-nm channels, where mobileions are regulated by the charged walls and the confined spacing. Insome embodiments, the ionic conductivity along the cellulose molecularchains in the solid-state metal-fibril complex can be at least 10⁻⁴ S/cm(e.g., on the order of 10⁻³ S/cm). For example, a fabricated example ofa solid-state Cu-cellulose complex with Li ions had an ionicconductivity of 5 mS/cm at room temperature, while a fabricated exampleof a solid-state Cu-cellulose complex with Na ions had an ionicconductivity of 0.1 mS/cm. Such values are significantly higher thanthat offered by conventional solid polymer electrolytes, which haveionic conductivities in the range of 10⁻⁵ to 10⁻⁸ S/cm at roomtemperature.

Fabrication of Solid-State Metal-Fibril Complexes

FIG. 3A shows an exemplary method 300 for fabricating a solid-statemetal-fibril complex from one or more elementary nanofibrils. The method300 can begin a process block 302, where a starting material for theelementary nanofibril(s) is prepared. As discussed above, in someembodiments, the elementary nanofibril(s) can be formed of anaturally-occurring polysaccharide, for example, cellulose, chitin,chitosan, or any combination thereof. The preparing 302 can thus includeobtaining a structure including the naturally-occurring elementarynanofibril(s) (e.g., piece of wood or other fibrous plant, exoskeletonof an insect, cell wall of fungi, shell of a shrimp, bacterial-producedcellulose fibers, etc.). In some embodiments, the preparing 302 caninclude modifying the structure in preparation for chemicalmodification. For example, in some embodiments, the structure is a pieceof wood or other fibrous plant, and the preparing 302 includes at leastone of delignification, densification, fibrillation, and shaping (e.g.,by twisting to form a cable). Alternatively or additionally, in someembodiments, the starting material may be in fiber form, each fiberincluding a plurality of the elementary nanofibril(s), and the preparing302 can include forming the starting material into a desired structure(e.g., paper, membrane, or a three-dimensional structure). For example,in some embodiments, a slurry containing the fibers can be formed into apaper using vacuum filtration and pressing.

The method 300 can proceed to process block 304, where a first metal isdissolved in an alkaline solution. For example, the alkaline solutioncan include sodium hydroxide (NaOH), potassium hydroxide (KOH), lithiumhydroxide (LiOH), or combinations thereof. The first metal can be anymetal capable of forming a coordination bond with the functional groupsof the polymer molecular chains, for example, Cu, Zn, Al, Ca, and/or Fe.

The method 300 can proceed to process block 306, where the elementarynanofibril(s) are immersed in an alkaline solution for a first timeperiod. For example, the alkaline solution can include sodium hydroxide(NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), orcombinations thereof. As described above, the immersion within thealkaline solution breaks the hydrogen bonds between functional groups(e.g., depotonation), thereby allowing the polymer molecular chains ofthe elementary nanofibril(s) to open up. The first time period may berelatively quickly, e.g., on the order of hours.

The method 300 can proceed to process block 308, where the immersion inthe alkaline solution is continued for a second time period, therebyforming a metal-fibril complex. As described above, the furtherimmersion within the alkaline solution allows the metal ions previouslydissolved in the alkaline solution (or otherwise added to the solutionduring the first and/or second time periods) to diffuse into the openedspace between the polymer molecular chains and to form a coordinationbond to the exposed functional groups of adjacent molecular chains. Thesecond time period may be relatively slow, for example, on the order ofdays (e.g., 1-2 weeks). In some embodiments, the alkaline solution maybe heated to help reduce the second time period.

Below a concentration threshold for the alkaline solution, the molecularstructure of elementary fibrils may not be changed. For example, foralkaline solution concentrations below 5% (w/v), a phase change to thedesired metal-fibril complex may not occur. In such situations, themetal ion will primarily coordinate among the cellulose nanofibrilsrather than within the cellulose nanofibrils (e.g., between the polymermolecular chains). Accordingly, in embodiments, the alkaline solutionemployed at process blocks 306-308 has a concentration between 5% (w/v)and the corresponding saturation concentration for the solution, forexample, ≥20% (w/v).

The method 300 can proceed to process block 310, where the metal-fibrilcomplex is immersed in a solvent having second ions therein. Asdescribed above, the metal coordination bonds form channels between theadjacent polymer molecular chains of the elementary nanofibril(s),thereby allowing the second ions to intercalate within the elementarynanofibril(s) (e.g., within the channels between the polymer molecularchains) by diffusion. For example, the second ions can be Li+, Na+, K+,Mg+, and/or proton (H+). Alternatively or additionally, the second ionscan include a molecule that donates a proton, such as ammonium ion(e.g., NH₄+).

In some embodiments, the second ions are provided in the alkalinesolution of process blocks 306-308, in which case process block 310 maybe considered an extension of 308. Alternatively, in some embodiments,the immersion of process block 310 is in a solvent different from thealkaline solution. For example, the solvent of process block 310 caninclude PC, EC, DMC, EMC, and/or DEC. To provide the desired secondions, an appropriate electrolyte or proton donor can be dissolved in thesolvent. For example, a Li-ion electrolyte can include LiClO₄, LiPF₆,LiBF₄, LiTFSI, and/or LiFSI. For example, a proton-donor can includeammonia, ammonium nitrate, ammonium chloride, ammonium sulfate,polyacrylic acid, and/or citric acid. Beyond those specific exampleslisted herein, other electrolytes and proton donors known in the art forthe desired second ions could also be used.

The method 300 can proceed to process block 312, where the metal-fibrilcomplex is immersed in an organic solvent. In some embodiments, thesecond ions of are provided in the organic solvent, in which caseprocess block 312 may be considered an extension of process block 310.Alternatively, in some embodiments, the immersion of process block 312is in a solvent different from the solvent of process block 310. Ineither case, the immersion of 312 can be effective to replace free watermolecules within the metal-fibril complex with molecules of the organicsolvent. In some embodiments, the organic solvent of treatment can be apolar aprotic solvent. For example, the organic solvent can include DMF,DMSO, PC, acetone, and/or EGDGE. In some embodiments, the organicsolvent can be selected to provide a desired crystal structure for thefinal solid-state metal-fibril complex. For example, use of DMSO and/orEGDGE as the organic solvent at process block 312 can yieldsubstantially crystalline morphology for cellulose-based complexes,while use of DMF, PC, and/or acetone as the organic solvent at processblock 312 can yield an amorphous morphology for cellulose-basedcomplexes.

The method 300 can proceed to process block 314, where the metal-fibrilcomplex is dried to form a solid-state ion-conducting structure. Inparticular, the drying is effective to evaporate the organic solventmolecules from the metal-fibril complex, thereby maintaining thenanostructure of the elementary nanofibril(s), e.g., with the metalcoordination bonds forming ion transport channels between the polymermolecular chains and second ions intercalated therein. The drying ofprocess block 314 can be effect to remove all or most of free water fromthe metal-fibril complex. For example, total water within themetal-fibril complex can be less than 10 wt %, and preferably boundwater within the metal-fibril complex is less than 8 wt %. The drying ofprocess block 314 can include vacuum drying, freeze drying, and/orcritical point drying.

Although process blocks 302-314 have been separately illustrated anddescribed as occurring once, practical implementation of the disclosedembodiments may employ multiple repetitions of a particular processblock before proceeding to the next process block. For example, thesecond ion immersion 310 may be repeated multiple times to ensuresufficient intercalation of the second ions within the metal-fibrilcomplex. In another example, the organic solvent immersion 312 may berepeated multiple times (or comprise a continuous washing with freshsolvent) to ensure sufficient removal of free water.

Moreover, although FIG. 3A illustrates a particular order for blocks302-314, embodiments of the disclosed subject matter are not limitedthereto. Indeed, in certain embodiments, the blocks may occur in adifferent order than illustrated or simultaneously with other blocks.For example, the metal dissolution of process block 304 can occur afterthe immersion during the first time period of process block 306 and/orat a same time as the immersion during the second time period of processblock 308.

FIG. 3B shows another exemplary method 350 for fabricating a solid-statemetal-fibril complex from one or more elementary nanofibrils. Processblocks 302-308 of method 350 may be substantially similar to processblocks 302-308 of method 300 in FIG. 3A. However, after process block308 and before introduction of any second ions into the metal-fibrilcomplex, method 350 can proceed to process block 352, where themetal-fibril complex is immersed in an organic solvent. Similar toprocess block 312 of method 300, the immersion of 352 in method 350 canbe effective to replace free water molecules within the metal-fibrilcomplex with molecules of the organic solvent.

The method 350 can proceed to process block 354, where the metal-fibrilcomplex is dried to form an intermediate solid-state metal-fibrilcomplex. Similar to process block 314 of method 300, the drying of 354is effective to evaporate the organic solvent molecules from themetal-fibril complex, thereby maintaining the nanostructure of theelementary nanofibril(s), e.g., with the metal coordination bondsforming ion transport channels between the polymer molecular chains.However, the dried metal-fibril complex lacks intercalated second ions.

To provide the desired second ions, the method 350 can proceed toprocess block 356, where the metal-fibril complex is immersed in asecond organic solvent. Similar to process block 310, the immersion of356 allows the second ions that are dissolved in the second organicsolvent to intercalate within the elementary nanofibril(s) (e.g., withinthe channels between the polymer molecular chains) by diffusion. Forexample, the second ions can be Li+, Na+, K+, Mg+, and/or proton (H+).Alternatively or additionally, the second ions can include a moleculethat donates a proton, such as ammonium ion (e.g., NH₄+). For example,the second organic solvent can include DMF, DMSO, PC, acetone, and/orEGDGE.

The method 350 can proceed to process block 358, where the metal-fibrilcomplex is dried to form the final solid-state ion-conducting structure.Similar to process block 314 of method 300, the drying of 358 iseffective to evaporate the second organic solvent molecules from themetal-fibril complex, thereby maintaining the nanostructure of theelementary nanofibril(s), e.g., with the metal coordination bondsforming ion transport channels between the polymer molecular chains andsecond ions intercalated therein.

Although process blocks 302-308 and 352-358 have been separatelyillustrated and described as occurring once, practical implementation ofthe disclosed embodiments may employ multiple repetitions of aparticular process block before proceeding to the next process block.For example, the second ion immersion 356 may be repeated multiple timesto ensure sufficient intercalation of the second ions within themetal-fibril complex. In another example, the organic solvent immersion352 and/or second organic solvent immersion 356 may be repeated multipletimes (or comprise a continuous washing with fresh solvent) to ensuresufficient removal of free water.

Moreover, although FIG. 3B illustrates a particular order for blocks302-308 and 352-358, embodiments of the disclosed subject matter are notlimited thereto. Indeed, in certain embodiments, the blocks may occur ina different order than illustrated or simultaneously with other blocks.For example, the metal dissolution of process block 304 can occur afterthe immersion during the first time period of process block 306 and/orat a same time as the immersion during the second time period of processblock 308.

Ion-Conducting Structures Including Solid-State Metal-Fibril Complexes

After either method 300 or method 350, the resulting solid-statemetal-fibril complex can be adapted for use as an ion-conductingstructure and/or ion-selective structure in a particular application.For example, the solid-state metal-fibril complex can be used as a solidelectrolyte, conductive additive or backbone, and/or ion conductivestructure (membrane, cable, etc.) in any type of electronic device orsystem, such as, but not limited to electrical energy storage devices(e.g., battery, supercapacitor, etc.), electrical power generationsystems (e.g., fuel cell, thermoelectric power generation device,osmotic power generation device), ion regulation or separation devices(e.g., cationic separation membrane, transistor), ion conductioncomponents (e.g., nanofluidic ion conductor, ion-conducting additive,solid-state electrolyte), and biological applications (e.g., ionregulation). The above list of applications for the solid-statemetal-fibril complex is not intended to be exhaustive. Indeed,application of the solid-state metal-fibril complex beyond thosespecifically listed above are also possible, and one of ordinary skillin the art will readily appreciate that the solid-state metal-fibrilcomplex could be adapted to other applications based on the teachings ofthe present disclosure.

In some embodiments, the solid-state metal-fibril complex can be formedas a substantially planar structure, for example, a membrane or sheethaving a thickness of 10 μm to 1000 μm, preferably 100 μm or less. Forexample, FIG. 4A shows an exemplary construction of a solid-statemetal-fibril complex as a sheet 400 with random orientation ofnanofibrils 402. Such a structure can result from chemical modification(e.g., performing method 300 or 350) of a paper starting material, forexample, commercially-manufactured paper or paper formed by vacuumfiltering and pressing of a slurry of the microfibrils and/ornanofibrils. The resulting sheet 400 can exhibits a smaller overalldimension than starting paper, for example, due to the collapse ofinternal pores (e.g., in the range of several microns to tens ofmicrons) within the paper during the chemical modification.Alternatively, the sheet 400 can be formed by first performing thechemical modification (e.g., performing method 300 or 350) on elementarynanofibrils and then forming the resulting solid-state metal-fibrilcomplex into a paper, for example, by vacuum filtering and pressing. Ineither case, due at least in part to the metal coordinate bonding, theresulting sheet 400 can be relatively stable under high pH aqueousconditions, unlike conventional paper that is unstable and tends toswell when exposed to such aqueous conditions.

FIGS. 4B-4C show additional exemplary constructions of solid-state metalfibril complexes as membranes with order arrangement of nanofibrils. Forexample, the membrane 410 in FIG. 4B can result from chemicalmodification (e.g., performing method 300 or 350) of vertically-cutwood, such that the membrane 410 inherits the aligned hierarchicalmicrostructure of the wood-starting material. Thus, the nanofibrils 412within membrane 410 can extend along and be aligned substantiallyparallel to a wood growth direction 414 and/or to major surfaces (e.g.,top and bottom surfaces) of the membrane 410. For example, the membrane420 in FIG. 4C can result from chemical modification (e.g., performingmethod 300 or 350) of horizontally-cut wood, such that the membrane 420inherits the aligned hierarchical microstructure of the wood-startingmaterial. Thus, the nanofibrils 422 within membrane 420 can extend alongand be aligned substantially parallel to a wood growth direction 414,but can be perpendicular to major surfaces (e.g., top and bottomsurfaces) of the membrane 420.

FIG. 4D shows another exemplary configuration of a solid-statemetal-fibril complex as a membrane 440 with ordered arrangement ofnanofibrils. For example, the membrane 440 can result from chemicalmodification (e.g., performing method 300 or 350) of a woven startingmaterial, such as a textile of fabric (e.g., cotton). The woven startingmaterial may include individual fibers 442 forming a regular array, eachof the fibers 442 being formed of an aggregate of constituent elementarynanofibrils that are aligned with and follow the orientation of theirassociate fiber 442. Alternatively, the membrane 440 can be formed byfirst performing the chemical modification (e.g., performing method 300or 350) on fibers 442 and then forming the weaving the solid-statemetal-fibril complex into the patterned arrangement of membrane 440.

FIGS. 4E-4F shows yet another exemplary configuration of a solid-statemetal-fibril complex as a single modified nanofibril 460. For example,the nanofibril 460 can result from chemical modification (e.g.,performing method 300 or 350) on a fibrillated starting material. Aswith the other configurations, the nanofibril 460 includes metal ions122 forming coordination bonds between functional groups of adjacentpolymer molecular chains 110 to form ion transport channels 462 throughthe nanofibril 460, with second ions 126 intercalated between thepolymer molecular chains 110 within the nanofibril 460. In someembodiments, the nanofibril 460 (or an aggregate of nanofibrils 460) canbe incorporated into other structures, components, or members to improvethe ion conducting properties thereof. For example, the nanofibril 460can be integrated with another material to form an electrode of anelectrical device.

Although particular shapes and fabrication techniques have beenillustrated in and discussed with respect to FIGS. 4A-4F, other shapesand fabrication techniques are also possible according to one or morecontemplated embodiments. Accordingly, the wood shapes and fabricationtechniques are not limited to those specifically illustrated.

Devices Including Ion-Conducting Solid-State Metal-Fibril ComplexStructures

In some embodiments, the solid-state metal-fibril complex can allowconstruction of a corresponding solid-state device, thereby avoidingpotential performance issues associated with aqueous versions of suchdevices. For example, in some embodiments, the solid-state metal-fibrilcomplex can be used as a solid-state electrolyte in a solid-statebattery. Alternatively or additionally, the solid-state metal-fibrilcomplex can be used as a conductive additive in one or both electrodes.As compared to conventional liquid-electrolyte batteries, thesolid-state battery can be safer, provide an increased energy density,and/or offer greater flexibility with electrode material selection. Forexample, solid-state batteries can be safer by avoiding leakage (e.g.,no liquid to leak), providing low flammability, and/or having improvedmechanical strength (e.g., due to the solid nature of the solid-statemetal-fibril complex). The higher packing density of the solid-statebattery can result in the improved energy density as compared toconventional liquid-electrolyte batteries. The electrochemical stabilityand fewer side reactions of the solid-state electrolyte can allow forbroad compatibility with various anodes and cathodes.

FIG. 5A shows an exemplary battery system 500 that can employ asolid-state ion-conducting metal-fibril complex. The battery system 500has a cathode 502 and an anode 506, each of which can be electricallycoupled to an electrical circuit 508 (e.g., load, voltage source) bycorresponding electrical connections 510. The battery system 500 can beconfigured as a solid-state system, with cathode 502 and anode 506 onopposite sides of and in contact with a separator membrane 504 that alsoacts a solid electrolyte. The separator membrane 504 can be orincorporate the solid-state metal-fibril complex, for example, having aconstruction as shown in any of FIGS. 4A-4D and/or fabricated accordingto the method of any of FIGS. 3A-3B. For example, the battery system 500can be constructed as lithium-ion battery, and the solid-statemetal-fibril complex of the separator membrane 504 can be a Cu-cellulosecomplex with Li ions intercalated therein. For example, the Cu-cellulosesolid-state ion conductor, either with an aligned cellulose structure(e.g., as in FIG. 4B or 4C) or isotropic cellulose (e.g., as in FIG.4A), can be processed into a thin dense layer (e.g., on the order of˜100 μm) so as to serve as the solid-state electrolyte in solid-statebattery system 500.

FIG. 5B shows another exemplary battery system 520 that can employ asolid-state ion-conducting metal-fibril complex. The battery system 520has a cathode 522 and an anode 526, each of which can be electricallycoupled to an electrical circuit 508 (e.g., load, voltage source) bycorresponding electrical connections 510. The battery system 520 can beconfigured as a solid-state system, with cathode 522 and anode 526 onopposite sides of and in contact with a separator membrane 504 that alsoacts a solid electrolyte. As with system 500 of FIG. 5A, the separatormembrane 504 can be or incorporate the solid-state metal-fibril complex,for example, having a construction as shown in any of FIGS. 4A-4D and/orfabricated according to the method of any of FIGS. 3A-3B. However, incontrast to system 500 of FIG. 5A, one or both of cathode 522 and anode526 can also include a solid-state ion-conducting metal-fibril complex.For example, cathode 522 and/or anode 526 can include a randomarrangement of elementary nanofibrils, each of which is a solid-statemetal-fibril complex having a construction as shown in FIGS. 4E-4Fand/or was fabricated according to the method of any of FIGS. 3A-3B. Thesolid-state metal-fibril complexes may be considered an additive to theelectrodes, with the elementary nanofibrils intermixed with theelectrode materials (e.g., base material), and may improve theion-conductivity of the respective electrode by at least an order ofmagnitude versus the electrode material alone.

FIG. 5C shows another exemplary battery system 540 that can employ asolid-state ion-conducting metal-fibril complex. Similar to the batterysystem 520 of FIG. 5B, the battery system 540 has a cathode 522 and ananode 526, each of which can be electrically coupled to an electricalcircuit 508 (e.g., load, voltage source) by corresponding electricalconnections 510, and one or both of the cathode 522 and/or anode 526includes a solid-state metal-fibril complex additive. However, insteadof separator membrane 504 as in FIGS. 5A-5B, battery system 540 has asolid-state electrolyte 542, which can be a conventional solid-stateelectrolyte. For example, solid-state electrolyte 542 can be anoxide-based electrolytes (e.g., garnet Li₇La₃Zr₂O₁₂, PerovskiteLi_(3.3)La_(0.56)TiO₃), sulfide-based electrolytes (e.g., Li₂S—P₂S₅,Li₁₀GeP₂S₁₂), or a polymer electrolytes (e.g., PEO, PVC, PMMA). Again,the solid-state metal-fibril complexes as additives to the cathode 522and/or anode 526 can improve the ion-conductivity thereof, for example,by at least an order of magnitude.

FIG. 5D shows yet another exemplary battery system 560 that can employ asolid-state ion-conducting metal-fibril complex. The battery system 560has a cathode 502 and an anode 506, each of which can be electricallycoupled to an electrical circuit 508 (e.g., load, voltage source) bycorresponding electrical connections 510. Between the cathode 502 andthe anode 506, a solid-state electrolyte 542 can be arranged. As withthe system of FIG. 5C, the solid-state electrolyte 542 can be any typeof conventional solid-state electrolyte. Alternatively, solid-stateelectrolyte 542 could instead be replaced with the separator membrane504 of FIGS. 5A-5B.

In addition, between the cathode 502 and the solid-state electrolyte542, a first solid electrolyte layer 564 can be disposed. In FIG. 5D,the electrolyte layer 564 is in contact with both the cathode 502 andthe solid-state electrolyte 542. However, in some embodiments, theelectrolyte layer 564 may be in contact with one or none of the cathode502 and the solid-state electrolyte 542, for example, due to one or moreintervening ion-conductive layers. Alternatively or additionally, asecond solid electrolyte layer 566 can be disposed between the anode 506and the solid-state electrolyte 542. In FIG. 5D, the electrolyte layer566 is in contact with both the anode 506 and the solid-stateelectrolyte 542. However, in some embodiments, the electrolyte layer 566may be in contact with one or none of the anode 506 and the solid-stateelectrolyte 542, for example, due to one or more interveningion-conductive layers. The first electrolyte layer 564 and/or the secondelectrolyte layer 566 can each include or be formed of solid-statemetal-fibril complexes, for example, having a construction as shown inany of FIGS. 4A-4D and/or fabricated according to the method of any ofFIGS. 3A-3B. Alternatively or additionally, the first electrolyte layer564 and/or the second electrolyte layer 566 can each include solid-statemetal-fibril complexes as an ion-conductive additives, for example,having a construction as shown in any of FIGS. 4E-4F.

FIG. 5E shows another exemplary battery system 580 that can employ asolid-state ion-conducting metal-fibril complex. Similar to the batterysystem 540 of FIG. 5C, the battery system 580 has a cathode 582 and ananode 586, each of which can be electrically coupled to an electricalcircuit 508 (e.g., load, voltage source) by corresponding electricalconnections 510, and the solid-state electrolyte 542 between the cathode582 and the anode 586 can be any type of conventional solid-stateelectrolyte or replaced with the separator membrane 504 of FIGS. 5A-5B.However, in contrast to FIG. 5C, one or both of the cathode 582 and/oranode 586 in system 580 includes an ion-conducting network 588 (alsoreferred to as matrix) formed by the solid-state metal-fibril complex.For example, the network 588 may have a plurality of woven fibers 442,similar to the construction of FIG. 4D. The network 588 can serve as asupport structure upon which the electrode particles 592 are dispersedon or embedded therein (e.g., in the narrow spaces between wovenfibers). Similar to the use of the solid-state metal-fibril complex asan additive to the electrodes, the use of ion-conducting network 588 canimprove the ion-conductivity of the resulting electrode versus theelectrode particles 592 alone.

In any of the embodiments of FIGS. 5A-5E, the battery system can beconstructed as a lithium ion battery. For example, the cathode can beformed of or include lithium cobalt oxide (LCO) (LiCoO₂), lithiummanganese oxide (LMO) (LiMn₂O₄), lithium iron phosphate (LFP)(LiFePO₄/C), lithium nickel cobalt manganese oxide (NMC) (LiNiCoMnO₂),lithium nickel manganese spinel (LNMO) (LiNi_(0.5)Mn_(1.5)O₄), lithiumnickel cobalt aluminum oxide (NCA) (LiNiCoAlO₂), and/or sulfur-carbon(S/C) composite. For example, the anode can be formed of or includegraphite, silicon, and/or carbon. Alternatively, the anode can include asolid piece of metal Li in contact with the separator membrane or thesolid-state electrolyte layer.

FIG. 6A shows an exemplary fuel cell system 600 that can employ anion-conducting metal-fibril complex. The fuel cell system 600 has acathode 602 and an anode 606, each of which can be electrically coupledto an electrical circuit 608 (e.g., load) by corresponding electricalconnections 610. For example, the cathode 602 and/or anode 606 can beformed of a metal, graphite, carbon composite, or carbon polymercomposite. One or both of the cathode 602 and anode 606 can include anappropriate catalyst. The cathode 602 and anode 606 can be on oppositesides of and in contact with proton exchange membrane (PEM) 604. Coupledto the cathode is a first manifold 612 that delivers an oxidizing agentto the cathode 602, for example, air or oxygen, and removes wasteproducts (e.g., water) therefrom. Coupled to the anode is anothermanifold 614 that delivers the chemical fuel to the anode 606, forexample, hydrogen gas or other supply of protons, and removes unusedfuel therefrom.

Fuel provided to the anode 606 by manifold 614 undergoes an oxidationreaction that generates protons and electrons. The protons move from theanode 606 to the cathode 602 via PEM 604, while the electrons move fromthe anode 606 to the cathode 602 via the external circuit (e.g.,electrical connections 610 and circuit 608), thereby producingelectrical power for use by circuit 608. The protons, electrons, andoxidizing agent react at the cathode 602 to form water, which is removedby manifold 612. At least PEM 604 of fuel cell system 600 can be orincorporate the disclosed metal-fibril complex, for example, having aconstruction as shown in any of FIGS. 4A-4D and/or fabricated accordingto the method of any of FIGS. 3A-3B. Alternatively or additionally, oneor both of the cathode 602 and anode 606 can include or be formed ofsolid-state metal-fibril complexes, for example, similar to theconstruction of electrodes in FIGS. 5B-5E.

The metal-fibril complex of PEM 604 can be initially formed as asolid-state component as described above. However, in operation, PEM 604may be exposed to relatively high humidity levels that would otherwiseraise the amount of motive water therein above the threshold for beingconsidered solid-state. Nevertheless, the initial structure has asolid-state construction and reverts to such construction when removedfrom the high humidity operational environment.

FIG. 6B shows an exemplary supercapacitor system 620 that can employ asolid-state ion-conducting metal-fibril complex. The supercapacitorsystem 620 has current collecting electrodes 628, 630, each of which canbe electrically coupled to an electrical circuit 634 (e.g., load,voltage source) by corresponding electrical connections 632. Between theelectrodes 628, 630 is disposed a pair of solid-electrolyte layers 622,626, with a separator membrane 624 disposed therebetween.

The separator membrane 624 of supercapacitor system 620 can be formed ofor incorporate the disclosed metal-fibril complex, for example, having aconstruction as shown in any of FIGS. 4A-4D and/or fabricated accordingto the method of any of FIGS. 3A-3B. Similarly, each of thesolid-electrolyte layers 622, 626 include or be formed of solid-statemetal-fibril complexes, for example, having a construction as shown inany of FIGS. 4A-4D and/or fabricated according to the method of any ofFIGS. 3A-3B.

Any of the features illustrated or described with respect to one of thesystems of FIGS. 5A-6B can be combined with any other of the systems ofFIGS. 5A-6B to provide other systems and embodiments not otherwiseillustrated or specifically described herein.

Aqueous Metal-Fibril Complexes

In some embodiments, the polymer molecular chains of one or moreelementary nanofibrils can be chemically modified to form an aqueousmetal-fibril complex. The aqueous metal-fibril complex can be formed ina similar manner as that described above with respect to the solid-statemetal-fibril complex, but without the solvent exchange and subsequent.Thus, as shown in FIG. 7, the aqueous metal-fibril complex 700 can havea similar structure and arrangement as the intermediate metal-fibrilcomplex of FIG. 2D.

The aqueous metal-fibril complexes 700 may enjoy similar properties andperformance advantages as the solid-state metal-fibril complexes. Forexample, the aqueous metal-fibril complexes can have high mechanicalstrength and solution stability. Moreover, the polymer molecular chains210 of the aqueous metal-fibril complex 700 provide ion transportchannels (e.g., ˜1 nm in diameter) with a relatively high surface area(e.g., ˜2400 m²/g). The aligned confinement and the weak attractionbetween the partially-hydrated ions and the channel walls (e.g., formedby adjacent polymer molecular chains 210) can lead to a low-friction andrapid flow. For example, the aqueous metal-fibril complex 700 can offera ˜10× enhancement in mobility for a range of ions, including Li+, Na+and K+. In some embodiments, the capability of the metal-fibril complexto promote fast and selective ion transport can overcome priortrade-offs between permeability and selectivity.

Fabrication of Aqueous Metal-Fibril Complexes

FIG. 8A shows an exemplary method 800 for fabricating an aqueousmetal-fibril complex from one or more elementary nanofibrils. The method800 can begin at process block 802, where a starting material for theelementary nanofibril(s) is prepared. For example, process block 802 caninclude by obtaining a structure including the naturally-occurringelementary nanofibril(s), modifying the structure in preparation forchemical modification, and/or forming the starting material into adesired structure, in a manner similar to that described in detail abovefor process block 302 in FIG. 3A.

The method 800 can proceed to process block 804, where a first metal isdissolved in an alkaline solution. For example, the alkaline solutioncan include NaOH, KOH, and/or LiOH, and the first metal can be any metalcapable of forming a coordination bond with the functional groups of thepolymer molecular chains, such as Cu, Zn, Al, Ca, and/or Fe. The method800 can proceed to process block 806, where the elementary nanofibril(s)are immersed in the alkaline solution for a first time period, in orderto break the hydrogen bonds between functional groups, thereby allowingthe polymer molecular chains of the elementary nanofibril(s) to open up.The method 800 can proceed to process block 808, where the immersion inthe alkaline solution is continued for a second time period, therebyforming a metal-fibril complex. For example, the further immersionallows the metal ions previously dissolved in the alkaline solution (orotherwise added to the solution during the first and/or second timeperiods) to diffuse into the opened space between the polymer molecularchains and to form a coordination bond to the exposed functional groupsof adjacent molecular chains. Thus, process blocks 804-808 can proceedin a manner similar to that described above for process blocks 304-308in FIG. 3A.

The method 800 can proceed to process block 810, where the metal-fibrilcomplex can optionally be immersed in a second solution having secondions therein. As described above, the metal coordination bonds formchannels between the adjacent polymer molecular chains of the elementarynanofibril(s), thereby allowing the second ions to diffuse within theelementary nanofibril(s) (e.g., within the channels between the polymermolecular chains), thereby forming the desired aqueous metal-fibrilcomplex with intercalated second ions. For example, the second solutionof process block 810 can include PC, EC, DMC, EMC, and/or DEC, and thesecond ions can be Li+, Na+, K+, Mg+, and/or proton (H+). Alternativelyor additionally, the second ions can include a molecule that donates aproton, such as ammonium ion (e.g., NH₄+). To provide the desired secondions, an appropriate electrolyte or proton donor can be dissolved in thesecond solution. For example, a Li-ion electrolyte can include LiClO₄,LiPF₆, LiBF₄, LiTFSI, and/or LiFSI, or a proton-donor can includeammonia, ammonium nitrate, ammonium chloride, ammonium sulfate,polyacrylic acid, and/or citric acid. Beyond those specific exampleslisted herein, other electrolytes and proton donors known in the art forthe desired second ions could also be used. Alternatively, the secondions can be provided in the alkaline solution of process blocks 806-808,in which case process block 810 may be considered an extension of 808 orotherwise omitted.

The method 800 can proceed to 812, where the aqueous metal-fibrilcomplex is adapted for use in a particular device or application. Suchapplications include electrical energy storage devices (e.g., battery,etc.), electrical power generation systems (e.g., thermoelectric powergeneration device, osmotic power generation device), ion regulation orseparation devices (e.g., cationic separation membrane, transistor), ionconduction components (e.g., nanofluidic ion conductor), and biologicalapplications (e.g., ion regulation). The above list of applications forthe aqueous metal-fibril complex is not intended to be exhaustive.Indeed, application of the aqueous metal-fibril complex beyond thosespecifically listed above are also possible, and one of ordinary skillin the art will readily appreciate that the aqueous metal-fibril complexcould be adapted to other applications based on the teachings of thepresent disclosure. In some embodiments, the aqueous metal-fibrilcomplex can be formed as a substantially planar structure, for example,a membrane or sheet having a thickness of 10 μm to 1000 μm, preferably100 μm or less. For example, the aqueous metal-fibril complex can haveconstructions similar to that illustrated in FIGS. 4A-4F for thesolid-state metal-fibril complexes.

Although process blocks 802-814 have been separately illustrated anddescribed as occurring once, practical implementation of the disclosedembodiments may employ multiple repetitions of a particular processblock before proceeding to the next process block. For example, thesecond ion immersion 810 may be repeated multiple times to ensuresufficient intercalation of the second ions within the metal-fibrilcomplex. Moreover, although FIG. 8 illustrates a particular order forblocks 802-814, embodiments of the disclosed subject matter are notlimited thereto. Indeed, in certain embodiments, the blocks may occur ina different order than illustrated or simultaneously with other blocks.For example, the metal dissolution of process block 804 can occur afterthe immersion during the first time period of process block 806 and/orat a same time as the immersion during the second time period of processblock 808.

Devices Including Ion-Conducting Aqueous Metal-Fibril Complex Structures

FIG. 9 shows an exemplary battery system 900 that can employ an aqueousion-conducting metal-fibril complex. The battery system 900 has acathode 904 and an anode 908, each of which can be electrically coupledto an electrical circuit 916 (e.g., load, voltage source) by respectiveelectrical contacts 910, 912 and corresponding electrical connections914. Cathode 904 and anode 908 can be on opposite sides of and incontact with a separator membrane 906, all of which can be disposed in acommon battery housing 902. The separator membrane 906 can be orincorporate the aqueous metal-fibril complex, for example, having aconstruction as shown in any of FIGS. 4A-4D and/or fabricated accordingto the method of FIG. 8. For example, a Cu-cellulose ion conductor,either with an aligned cellulose structure (e.g., as in FIG. 4B or 4C)or isotropic cellulose (e.g., as in FIG. 4A), can be processed into athin dense layer (e.g., on the order of ˜100 μm) so as to serve as theaqueous electrolyte in battery system 900.

One or both of the cathode 904 and/or anode 908 in system 900 caninclude an aqueous ion-conducting network 918 (also referred to as amatrix) formed by the aqueous metal-fibril complex. For example, thenetwork 918 may have a plurality of woven fibers 920, similar to theconstruction of FIG. 4D. The network 918 can serve as a supportstructure upon which the electrode particles 922 are dispersed on orembedded therein (e.g., in the narrow spaces between woven fibers).Similar to the use of the solid-state metal-fibril complex as anadditive to the electrodes, the use of ion-conducting network 918 canimprove the ion-conductivity of the resulting electrode versus theelectrode particles 922 alone. Alternatively, one or both of the cathode904 and/or anode 908 can include aqueous metal-fibril complex as anadditive to the electrode materials, for example, in a manner similar tothat described above for solid-state metal-fibril complex additives inthe electrodes of FIGS. 5B-5D.

For example, the battery system 900 of FIG. 9 can be constructed as alithium ion battery. For example, the cathode can be formed of orinclude LCO, LMO, LFP, NMC, LNMO, NCA, and/or S/C composite. Forexample, the anode can be formed of or include graphite, silicon, and/orcarbon. Alternatively, the anode can include a solid piece of metal Liin contact with the separator membrane or the solid-state electrolytelayer. For example, each of the aqueous metal-fibril complexes (e.g., inseparator membrane 906 and/or electrodes 904, 908) can have Li ionsintercalated therein.

Any of the features illustrated or described with respect to one of thesystems of FIGS. 5A-6B and 9 can be combined with any other of thesystems of FIGS. 5A-6B and 9 to provide other systems and embodimentsnot otherwise illustrated or specifically described herein.

Ion-Conducting Delignified Wood Structures

In some embodiments, the polymer molecular chains of one or moreelementary nanofibrils can be chemically modified without forming metalcoordination bonds between the functional groups of the polymermolecular chains. In some embodiments, aligned nanochannels can beformed between the elementary nanofibril, for example, cellulosenanofibrils produced from a wood structure. As discussed above andillustrated in FIG. 10A, a wood structure 1000 is primarily composed ofcellulose nanofibrils 1006, hemicellulose, and lignin 1002, with thethree components intertwining with each other to form a strong and rigidwall structure. The cellulose nanofibrils 1006 are substantially alignedalong the wood growth direction, and the resulting ion-conductingstructure can inherit this aligned microstructure after chemicalmodification. In some embodiments, a delignification treatment 1004 isperformed on the wood structure 1000, thereby partially or fullyremoving lignin 1002 from the structure in order to expose the cellulosenanofibrils 1006, as shown in FIG. 10B. The delignification treatment1004 may also simultaneously remove some, most, or all of thehemicellulose in the wood structure 1000, thereby yielding a structure1010 comprised substantially of cellulose. For example, afterdelignification, the cellulose structure 1010 can have a hierarchicalarrangement, with spacing between cellulose fiber bundles being about 30nm, spacing between elementary nanofibrils 1006 within the bundles beingabout 2 nm, and spacing between cellulose molecular chains within thenanofibrils 1006 being about 0.7 nm.

Due to the dissociation of the surface functional groups, the chargedsurface of cellulose nanofibrils 1020 can attract layers of counter-ionsadjacent to the nanofibrils, with an exponentially decaying ionconcentration toward the center of an ion transport channel 1022, asshown in FIG. 10C. The interface-dominated electrostatic fieldsurrounding the cellulose nanofibrils 1020 thus provides regulated iontransport along the fiber direction 1028. In addition, the surfacecharge, the geometry, and/or the molecular structure of cellulose can betuned to modify the ion regulation capability of the resultingstructure. In some embodiments, the charge density and/or charge type ofthe functional groups of the polymer molecular chains can be modified byappropriate chemical treatment 1012. For example, in some embodiments,the elementary nanofibrils 1006 can be subjected to a TEMPO treatment toconvert hydroxyl functional groups to carboxyl groups. Alternatively, insome embodiments, the elementary nanofibrils 1006 can be subjected to aCHPTAC treatment to convert the surface charge of the functional groupsfrom negative to positive.

Fabrication of Ion-Conducting Delignified Wood Structures

FIG. 11 shows an exemplary method 1100 for fabricating an ion-conductingcellulose structure from partially or fully delignified wood. The method1100 can begin at process block 1102, where a starting wood material isprepared. For example, the preparing 1102 can include obtaining apre-cut piece of wood with desired orientation or cutting a piece ofwood to have a desired orientation (e.g., horizontal or rotationalcutting perpendicular to the wood growth direction, vertical cuttingparallel to the wood growth direction, and/or cutting at any anglecrossing the wood growth direction).

The method 1100 can proceed to process block 1104, where the initialpiece of wood is subject to partial delignification (e.g., 95% or lessof lignin removed) or full delignification (e.g., at least 95% of ligninremoved) in order to expose the cellulose nanofibrils of the wood. Insome embodiments, delignification can be achieved by immersing the woodpiece in a solution comprising chemicals used in pulping or pulpbleaching. For example, the chemical solution for delignification caninclude at least one of NaOH, NaOH/Na₂S, NaHSO₃+SO₂+H₂O, NaHSO₃,NaHSO₃+Na₂SO₃, NaOH+Na₂SO₃, Na₂SO₃, NaOH+AQ, NaOH/Na₂S+AQ,NaHSO₃+SO₂+H₂O+AQ, NaOH+Na₂SO₃+AQ, NaHSO₃+AQ, NaHSO₃+Na₂SO₃+AQ,Na₂SO₃+AQ, NaOH+Na₂S+Na₂S_(n), Na₂SO₃+NaOH+CH₃OH+AQ, CH₃OH, C₂H₅OH,C₂H₅OH+NaOH, C₄H₉OH, HCOOH, CH₃COOH, CH₃OH+HCOOH, C₄H₈O₂, NH₃.H₂O,p-TsOH, H₂O₂, NaClO, NaClO₂+acetic acid, ClO₂, and Cl₂, where n in aninteger and AQ is Anthraquinone. In some embodiments, the chemicaltreatment for delignification can be performed under vacuum, so as toencourage the chemical solution to fully penetrate the cell walls andlumina of the wood.

In some embodiments, the delignification of process block 1104 cancomprise a single step chemical treatment, e.g., a single exposure to asingle chemical or mixture of chemicals (e.g., a bath of H₂O₂).Alternatively, the chemical treatment for delignification can be amulti-step chemical treatment, e.g., a first exposure to a firstchemical or mixture (e.g., a bath of NaOH and Na₂SO₃) followed by asecond exposure to a second chemical or mixture (e.g., a bath of H₂O₂,2.5 mol/L), for example, to ensure complete removal of lignin and/orhemicellulose. Further details regarding exemplary processes andmaterials for delignification are described in, for example, theInternational Publication Nos. WO 2018/191181, WO 2018/187238, and WO2019/055789, incorporated by reference above.

The method 1100 can proceed to process block 1106, where the chargedensity and/or charge type of functional groups of the cellulosemolecular chains are modified by chemical treatment. For example, insome embodiments, a TEMPO treatment is used to convert hydroxylfunctional groups of the molecular chains in the exposed cellulosenanofibrils to carboxyl groups. Alternatively, in some embodiments, aCHPTAC treatment is used to convert the surface charge of the functionalgroups of the molecular chains in the exposed cellulose nanofibrils fromnegative charge to positive charge (e.g., quaternized cellulose). Forexample, in a mixed solution containing NaOH, urea, and distilled water,the quaternized cellulose was synthesized via a reaction between epoxideand cellulose sodium alkoxide. After chemical treatment with CHPTAC, thenatural cellulose is converted into quaternized cellulose, whichpresents a positive charge in solution. Compared with the molecularstructure of native cellulose, the resulting chemically-modifiedcellulose structure presents cationic functional groups (e.g.,—(CH3)₃N₊) via the extended side chain of cellulose.

The method 1100 can proceed to process block 1108, where thechemically-modified cellulose structure (e.g., membrane or paper) can befilled with an electrolyte for use as an ion conducting or ion selectivestructure (e.g., an ion separating device). For example, thechemically-modified cellulose structure can be filled with aqueouselectrolyte (e.g., KCl, NaCl, etc.) or a polymer electrolyte (e.g.,NaOH-based polymer electrolyte). In some embodiments, process block 1108can further include filling the chemically-modified cellulose structurewith a polymer or epoxy, for example, to block micro-sized channelswithin the cellulose structure while leaving nano-scale channelsotherwise open for ion selection.

Although process blocks 1102-1108 have been separately illustrated anddescribed as occurring once, practical implementation of the disclosedembodiments may employ multiple repetitions of a particular processblock before proceeding to the next process block. Moreover, althoughnot separately illustrated in FIG. 11 or discussed above, method 1100may also include rinsing or other intermediate processing steps betweenillustrated process blocks 1102-1108.

Devices Including Ion-Conducting Delignified Wood Structures

FIG. 12 shows an exemplary thermoelectric device 1200 that can employ anion-conducting structure derived from delignified wood. Thethermoelectric device 1200 has a pair of electrodes 1202, 1206 connectedto a circuit 1212 (e.g., load, energy storage device) by correspondingelectrical connections. An ion-conducting membrane 1204 can be disposedbetween the electrodes 1202, 1206 and can be infiltrated withelectrolyte, for example a polymer electrolyte (e.g., NaOH-based polymerelectrolyte). The ion-conducting membrane 1204 can be achemically-modified cellulose-based ion conducting structure fabricatedfrom wood, for example, using the method described above with respect toFIG. 11. For example, the cellulose-based membrane 1204 can be producedby extracting the lignin and hemicellulose from natural wood that hasbeen vertically cut. After delignification, the naturally-alignedcellulose nanofibrils are retained and feature a negatively-chargedsurface. This negative charge can be further enhanced by TEMPOoxidation.

The cellulosic membrane 1204 relies on nanoscale confinement of theoxidized, aligned cellulose molecular chains to promote ionicselectivity, which can enhance thermoelectric performance. When athermal gradient (T) is applied across the membrane 1204 (e.g., in adirection parallel to the ion transport channels therein), a Seebeckcoefficient can be generated that exceeds that of the bulk electrolyte.The Seebeck coefficient can originate from the ionic selectivity of thenegatively-charged cellulose nanofibers and the resulting development ofsurface-charge-governed ion transport (e.g., from electrode 1202,through membrane 1204, to electrode 1206), where a natural asymmetry interms of the number density of positive and negative ions occurs withinthe nanoscale ion transport channels.

FIG. 13 shows an exemplary transistor 1300 that can employ anion-conducting structure derived from delignified wood. The transistor1300 can have a pair of electrolyte reservoirs 1304, 1306 (e.g., KCl)connected by an intervening ion-conducting membrane 1302. Theion-conducting membrane 1302 can also be filled with electrolyte (e.g.,liquid electrolyte, such as KCl). The electrolyte reservoirs 1304, 1306can have respective electrodes 1312, 1314 (e.g., Ag/AgCl electrodes)disposed therein to connect the reservoirs to a circuit 1316 (e.g.,load, voltage source). A metal contact 1308 (e.g., silver film or paste)can be disposed on a surface of the ion-conducting membrane 1302 and canact as a gate for the transistor 1300. A voltage source 1310 can beelectrically connected to the gate contact 1308 to modulate operation ofthe transistor 1300. The ion-conducting membrane 1302 can be achemically-modified cellulose-based ion conducting structure fabricatedfrom wood, for example, using the method described above with respect toFIG. 11.

FIG. 14 shows an exemplary osmotic power generation system 1400 that canemploy an ion-conducting structure derived from delignified wood. Thepower generation system 1400 can have a pair of electrolyte reservoirs1404, 1406 (e.g., NaCl) connected by an intervening ion-conductingmembrane 1402. The electrolyte reservoirs 1404, 1406 can have respectiveelectrodes 1410, 1412 (e.g., Ag/AgCl electrodes) disposed therein toconnect the reservoirs to a circuit 1414 (e.g., load, power storage). Anelectrolyte concentration of one of the reservoirs 1404, 1406 can besubstantially greater (e.g., at least 10×, 100×, or 1000×) than theelectrolyte concentration in the other of the reservoirs 1404, 1406. Forexample, reservoir 1404 can contain seawater having a concentration of100 mmol/L of NaCl, while reservoir 1406 can contain fresh water havinga concentration of 0.001 mmol/L of NaCl.

The ion-conducting membrane 1402 can be a chemically-modifiedcellulose-based ion conducting structure fabricated from wood, forexample, using the method described above with respect to FIG. 11. Forexample, the cellulose-based membrane 1402 can be produced by extractingthe lignin and hemicellulose from natural wood that has been verticallycut. After delignification, the naturally-aligned cellulose nanofibrilsare retained and feature a negatively-charged surface. In someembodiments, the ion-conducting membrane 1402 can be infiltrated withepoxy, for example, to block micro-sized lumens inherited from theoriginal wood structure. The resulting polymer-filled membrane 1402 hasaligned nanoscale channels along the wood growth direction that canremain open and that provide cation-selective fluidic pathways withnegative surface charge due to the dissociation of the hydroxyl groupsof the cellulose molecular chains. When exposed to the electrolyteconcentration difference between the reservoirs 1404, 1406, anelectrical double layer formed along the nanocellulose of the membrane1402 allows the cations to efficiently pass through the opennanochannels while impeding the transport of anions, therebyestablishing an electrical potential in an opposite direction as thecation movement direction.

Any of the features illustrated or described with respect to one of thesystems of FIGS. 5A-6B, 9, and 12-14 can be combined with any other ofthe systems of FIGS. 5A-6B, 9, and 12-14 to provide other systems andembodiments not otherwise illustrated or specifically described herein.

Ion-Conducting Densified Wood Structures

In some embodiments, cellulose structures formed from wood can bedensified before or after chemical modification to form ion-conductingstructures. As discussed above and illustrated in FIG. 15A, a woodstructure 1500 has an aligned microstructure that would be retainedafter the chemical modification. In particular, the microstructureincludes large size pores 1502 and smaller size wood cell lumen 1504that extend along the wood growth direction 1506. Densification can thusbe used to remove the micro-size pores 1502 and lumen 1504, as shown inFIG. 15B, thereby ensuring the nanoscale ion transport channels producedby the chemical modification are the preferred transport path for ions.

For example, a chemical treatment 1510 (e.g., TEMPO or CHPTAC) can beused to modify functional groups of the cellulose molecular chains. Thechemically modified wood can then be subjected to densification 1512,for example, by pressing in a direction crossing the wood growthdirection 1506, such that a thickness 1508 of the wood is reduced. Forexample, the densification may be such that a final thickness 1522 hasbeen reduced by at least 75% (e.g., by at least 90%) as compared to theoriginal thickness 1508. The densification 1512 can reduce the amount ofspace between the wood cellulose fiber channels (e.g., pores 1502, lumen1504), thereby removing larger diameter pathways that exceed the Debyelength and ensuring the membrane's dense structure, high strength, andhigh ionic conductivity.

In some embodiments, the chemical treatment 1510 can directly modify thecellulose and hemicellulose of wood using CHPTAC via etherification,thereby introducing cationic ions (CH₃)₃N⁺ (e.g., at 1528) onto thesurface of the nanofluidic channels between cellulose molecular chains1524 for transport of ions 1526 therein. The resultingchemically-modified wood structure can thus act a cationic woodmembrane.

Fabrication of Ion-Conducting Densified Wood Structures

FIG. 16 shows an exemplary method 1600 for fabricating an ion-conductingcellulose structure from densified wood. The method 1600 can begin atprocess block 1602, where a starting wood material is prepared. Forexample, the preparing 1602 can include obtaining a pre-cut piece ofwood with desired orientation or cutting a piece of wood to have adesired orientation (e.g., horizontal or rotational cuttingperpendicular to the wood growth direction, vertical cutting parallel tothe wood growth direction, and/or cutting at any angle crossing the woodgrowth direction).

The method 1600 can proceed to process block 1604, where the chargedensity and/or charge type of functional groups of the cellulosemolecular chains in the initial wood are modified by chemical treatment.For example, in some embodiments, a TEMPO treatment is used to converthydroxyl functional groups of the molecular chains in the exposedcellulose nanofibrils to carboxyl groups. Alternatively, in someembodiments, a CHPTAC treatment is used to convert the surface charge ofthe functional groups of the molecular chains in the exposed cellulosenanofibrils from negative charge to positive charge (e.g., quaternizedcellulose). For example, in a mixed solution containing NaOH, urea, anddistilled water, the quaternized cellulose was synthesized via areaction between epoxide and cellulose sodium alkoxide. After chemicaltreatment with CHPTAC, the natural cellulose is converted intoquaternized cellulose, which presents a positive charge in solution.Compared with the molecular structure of native cellulose, the resultingchemically-modified cellulose structure presents cationic functionalgroups (e.g., —(CH₃)₃N+) via the extended side chain of cellulose.

The method 1600 can proceed to process block 1606, where thechemically-modified wood can optionally be subject to densification. Forexample, the wood can be densified by pressing in a direction crossingthe wood growth direction, such that a thickness of the wood is reduced.The densification may eliminate, or at least reduce, micro-scale orlarger spaces within the chemically-modified wood, while retaining flowpaths that are less the Debye length. In some embodiments, thedensification of process block 1606 can be performed withoutdelignification of the wood structure. In other embodiments,delignification may be performed prior to or after densification.Further details regarding exemplary densification processes aredescribed in, for example, International Publication Nos. WO 2018/191181and WO 2019/055789, which were incorporated by reference above.

The method 1600 can proceed to process block 1608, where thechemically-modified, densified cellulose structure (e.g., membrane orpaper) can be filled with an electrolyte for use as an ion conducting orion selective structure. For example, the chemically-modified, densifiedcellulose structure can be filled with aqueous electrolyte (e.g., KCl,NaCl, etc.) or a polymer electrolyte (e.g., NaOH-based polymerelectrolyte).

Although process blocks 1602-1608 have been separately illustrated anddescribed as occurring once, practical implementation of the disclosedembodiments may employ multiple repetitions of a particular processblock before proceeding to the next process block. Moreover, althoughnot separately illustrated in FIG. 16 or discussed above, method 1600may also include rinsing or other intermediate processing steps betweenillustrated process blocks 1602-1608.

Devices Including Ion-Conducting Densified Wood Structures

In some embodiments, a chemically-modified, densified wood structure canbe employed as an ion selective membrane, for example, a cationicmembrane. The cationic membrane can be fabricated from wood, forexample, using the method described above with respect to FIG. 16. Forexample, the cellulose and hemicellulose of wood can be chemicallymodified using CHPTAC via etherification, which introduces cationic ion—(CH₃)₃N⁺ onto the surface of the nanofluidic wood channels for ionictransport for the first time. Subsequent densification eliminate largerchannels in the wood structure in favor of a large number of nanofluidicchannels. When situated between electrolyte reservoirs (for example, ina configuration similar to that illustrated in FIG. 14), the cationicmembrane can facilitate the transport of negatively charged ionstherethrough while preventing, or at least inhibiting, the passage ofpositively charged ions. The cationic wood membrane can thus provideefficient ion conductance via the large number of nanofluidic channels.Any of the features illustrated or described with respect to one of thesystems of FIGS. 5A-6B, 9, 12-14, and the above-described cationicmembrane can be combined with any other of the systems of FIGS. 5A-6B,9, 12-14 and above-described cationic membrane to provide other systemsand embodiments not otherwise illustrated or specifically describedherein.

REPRESENTATIVE EMBODIMENTS

Certain representative embodiments are exemplified in the followingnumbered clauses:

1. An ion-conducting structure, comprising a metal-fibril complex formedby one or more elementary nanofibrils, each elementary nanofibril beingcomposed of a plurality of cellulose molecular chains with functionalgroups, each elementary nanofibril having a plurality of metal ions,each metal ion acting as a coordination center between the functionalgroups of adjacent cellulose molecular chains so as to form a respectiveion transport channel between the cellulose molecular chains, whereinthe metal-fibril complex comprises a plurality of second ions, eachsecond ion being disposed within one of the ion transport channels so asto be intercalated between the corresponding cellulose molecular chains,and wherein the metal-fibril complex is a solid-state structure.

2. The ion-conducting structure of clause 1, wherein the metal-fibrilcomplex further comprises polysaccharide, poly(vinyl chloride) (PVC),poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), poly(ethyleneoxide) (PEO), poly(acrylonitrile) (PAN), poly(ethyl methacrylate)(PEMA), poly(methyl methacrylate) (PMMA), poly(ethylene terephthalate)(PET), polyethylene (PE), poly(ethylene naphthalate) (PEN), polyamide(PA), poly(vinylidene chloride) (PVDC), polylactic acid (PLA), orcombinations thereof.

3. The ion-conducting structure of any of clauses 1-2, wherein theplurality of metal ions comprises copper (Cu), zinc (Zn), aluminum (Al),calcium (Ca), iron (Fe), or combinations thereof.

4. The ion-conducting structure of any of clauses 1-3, wherein theplurality of second ions comprises lithium (Li+), sodium (Na+),potassium (K+), magnesium (Mg+), protons (H+), or combinations thereof.

5. The ion-conducting structure of any of clauses 1-4, wherein a widthof each ion transport channel is about 1 nm, and a spacing betweenadjacent ion transport channels within each elementary nanofibril isless than 2 nm.

6. The ion-conducting structure of any of clauses 1-5, wherein eachelementary nanofibril comprises at least ten cellulose molecular chains,preferably, 12-36 cellulose molecular chains, inclusive.

7. The ion-conducting structure of any of clauses 1-6, wherein themetal-fibril complex has a plurality of the elementary nanofibrils andis formed as a sheet, film, or membrane.

8. The ion-conducting structure of any of clauses 1-7, wherein thesheet, film, or membrane has a thickness between 10 μm and 1000 μm,inclusive.

9. The ion-conducting structure of any of clauses 1-8, wherein themetal-fibril complex has a conductivity of at least 10⁻⁴ S/cm.

10. The ion-conducting structure of any of clauses 1-9, wherein a totalcontent of water within the metal-fibril complex is less than or equalto 10 wt %.

11. The ion-conducting structure of any of clauses 1-10, wherein acontent of bound water within the metal-fibril complex is less than orequal to 8 wt %.

12. A device comprising an electrode or ion-conducting member, theelectrode or ion-conducting member having the ion-conducting structureof any of clauses 1-11 dispersed therein as a conductive additive.

13. The device of clause 12, wherein the device is constructed as abattery, a fuel cell, a supercapacitor, a transistor, a thermal powerharvesting device, an electricity generating device, or an ionseparating device.

14. A device comprising a membrane, the membrane comprising theion-conducting structure of any of clauses 1-11.

15. The device of clause 14, wherein the device is constructed as abattery, a fuel cell, a supercapacitor, a transistor, a thermal powerharvesting device, an electricity generating device, or an ionseparating device.

16. A battery comprising: first and second electrodes; and a separatormembrane between the first and second electrodes, the separator membranecomprising a solid-state metal-fibril complex, wherein one of the firstand second electrodes operates as a cathode and the other of the firstand second electrodes operates as an anode, the solid-state metal-fibrilcomplex is formed by a plurality of first nanofibrils, each firstnanofibril being composed of a plurality of cellulose molecular chainswith first functional groups, each first nanofibril having a pluralityof first metal ions, each first metal ion acting as a first coordinationcenter between the first functional groups of adjacent cellulosemolecular chains so as to form a respective first ion transport channelthrough the separator membrane, and the solid-state metal-fibril complexcomprises a plurality of second ions, each second ion being disposedwithin one of the first ion transport channels so as to be intercalatedbetween the corresponding cellulose molecular chains.

17. The battery of clause 16, wherein: the first electrode, the secondelectrode, or both the first and second electrodes comprise a basematerial and an additive interspersed within the base material, theadditive comprises one or more second nanofibrils, each secondnanofibril being composed of a plurality of second cellulose molecularchains with second functional groups, each second nanofibril having aplurality of second metal ions, each second metal ion acting as a secondcoordination center between the second functional groups of adjacentsecond cellulose molecular chains so as to form a respective second iontransport channel between the second cellulose molecular chains, eachsecond nanofibril comprising a plurality of third ions, each third ionbeing disposed within a respective one of the second ion transportchannels so as to be intercalated between the corresponding secondcellulose molecular chains.

18. A battery comprising: first and second electrodes, one of the firstand second electrodes operating as a cathode and the other of the firstand second electrodes operating as an anode; and a separator between thefirst and second electrodes, the separator comprising a solid-stateelectrolyte, the first electrode, the second electrode, or both thefirst and second electrodes comprise a solid-state metal-fibril complex,the solid-state metal-fibril complex is formed by a plurality ofnanofibrils, each nanofibril being composed of a plurality of cellulosemolecular chains with functional groups, each nanofibril having aplurality of metal ions, each metal ion acting as a coordination centerbetween the functional groups of adjacent cellulose molecular chains soas to form a respective ion transport channel between the cellulosemolecular chains, and the solid-state metal-fibril complex comprises aplurality of second ions, each second ion being disposed within one ofthe ion transport channels so as to be intercalated between thecorresponding cellulose molecular chains.

19. The battery of clause 18, wherein the solid-state electrolytecomprises an oxide-based electrolyte, a sulfide-based electrolytes, apolymer electrolyte, or combinations thereof.

20. The battery of one of clauses 16-19, wherein the solid-statemetal-fibril complex, the additive, or both the solid-state metal-fibrilcomplex and the additive further comprise polysaccharide, poly(vinylchloride) (PVC), poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA),poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(ethylmethacrylate) (PEMA), poly(methyl methacrylate) (PMMA), poly(ethyleneterephthalate) (PET), polyethylene (PE), poly(ethylene naphthalate)(PEN), polyamide (PA), poly(vinylidene chloride) (PVDC), polylactic acid(PLA), or combinations thereof.

21. The battery of one of clauses 16-20, wherein: each metal ioncomprises copper (Cu), zinc (Zn), aluminum (Al), calcium (Ca), iron(Fe), or combinations thereof; and/or the second ions, the third ions,or both the second ions and the third ions comprise lithium (Li), sodium(Na), potassium (K), magnesium (Mg), or combinations thereof.

22. The battery of one of clauses 16-21, wherein: the first electrodeoperates as the anode and comprises graphite, silicon, carbon, orcombinations thereof; and/or the second electrode operates as thecathode and comprises lithium cobalt oxide (LCO) (LiCoO₂), lithiummanganese oxide (LMO) (LiMn₂O₄), lithium iron phosphate (LFP)(LiFePO₄/C), lithium nickel cobalt manganese oxide (NMC) (LiNiCoMnO₂),lithium nickel manganese spinel (LNMO) (LiNi_(0.5)Mn_(1.5)O₄), lithiumnickel cobalt aluminum oxide (NCA) (LiNiCoAlO₂), sulfur-carbon (S/C)composite, or combinations thereof.

23. The battery of one of clauses 16-22, wherein: each of the first andsecond electrodes is in contact with the separator membrane; and theseparator membrane is constructed to operate as a solid-stateelectrolyte between the first and second electrodes.

24. The battery of one of clauses 16-23, wherein the separator membraneor the solid-state electrolyte has a thickness between 10 μm and 1000μm, inclusive.

25. A method, comprising: (a) forming a metal-fibril complex byimmersing a plurality of elementary nanofibrils within an alkalinesolution having a concentration of at least 5% (w/v) and a plurality ofmetal ions dissolved therein, each elementary nanofibril being composedof a plurality of cellulose molecular chains with functional groups, theimmersing being such that hydrogen bonds between adjacent functionalgroups of the cellulose molecular chains are broken so as to expose thefunctional groups and such that the dissolved metal ions from thealkaline solution form coordination bonds with the exposed functionalgroups; (b) after (a), intercalating second ions between adjacentcellulose molecular chains of the metal-fibril complex by immersing themetal-fibril complex in a first solution having a plurality of thesecond ions dissolved therein; (c) after (a), replacing free water inthe metal-fibril complex by immersing the metal-fibril complex in anorganic solvent; and (d) after (c), drying the metal-fibril complex suchthat a total content of water within the metal-fibril complex is lessthan or equal to 10 wt %, thereby forming the metal-fibril complex withintercalated second ions as a solid-state ion conducting structure,wherein: (i) the first solution is the organic solvent, and theintercalating of (b) and the replacing free water of (c) are performedsimultaneously, or (ii) the first solution is separate from the organicsolvent, and the intercalating of (b) is performed before or after thereplacing free water of (c).

26. The method of clause 25, wherein the elementary nanofibrils furthercomprise polysaccharide, poly(vinyl chloride) (PVC), poly(vinyl alcohol)(PVA), poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), poly(ethyl methacrylate) (PEMA), poly(methylmethacrylate) (PMMA), poly(ethylene terephthalate) (PET), polyethylene(PE), poly(ethylene naphthalate) (PEN), polyamide (PA), poly(vinylidenechloride) (PVDC), polylactic acid (PLA), or combinations thereof.

27. The method of any of clauses 25-26, wherein the plurality of metalions comprises copper (Cu), zinc (Zn), aluminum (Al), calcium (Ca), iron(Fe), or combinations thereof.

28. The method of any of clauses 25-27, wherein the plurality of secondions comprises lithium (Li), sodium (Na), potassium (K), magnesium (Mg),protons (H+), or combinations thereof.

29. The method of any of clauses 25-28, wherein the alkaline solutioncomprises sodium hydroxide (NaOH), potassium hydroxide (KOH), lithiumhydroxide (LiOH), or combinations thereof.

30. The method of any of clauses 25-29, wherein the first solutioncomprises propylene carbonate (PC), ethylene carbonate (EC), dimethylcarbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC),or combinations thereof.

31. The method of any of clauses 25-30, wherein the organic solventcomprises dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylenecarbonate (PC), acetone, ethylene glycol diglycidyl ether (EGDGE), orcombinations thereof.

32. The method of any of clauses 25-31, wherein the organic solventcomprises DMSO, EGDGE, or combinations thereof, and, after (c), thecellulose molecular chains have a crystalline morphology.

33. The method of any of clauses 25-32, wherein the organic solventcomprises DMF, PC, acetone, or combinations thereof, and, after (c), thecellulose molecular chains have an amorphous morphology.

34. The method of any of clauses 25-33, wherein, prior to (a), eachelementary nanofibril has a diameter less than 5 nm and comprises atleast ten cellulose molecular chains, preferably 12-36 cellulosemolecular chains.

35. The method of any of clauses 25-34, wherein, after (a), iontransport channels are formed between adjacent cellulose molecularchains by the metal ions acting as coordination centers between thefunctional groups, a width of each ion transport channel is about 1 nm,and a spacing between adjacent ion transport channels is less than 2 nm.

36. The method of any of clauses 25-35, further comprising, prior to(a), subjecting a parent structure containing the elementary nanofibrilsto a mechanical fibrillation process, a chemical fibrillation process,an enzymatic fibrillation process, or combinations thereof, so as toexpose the nanofibrils from the parent structure.

37. The method of any of clauses 25-36, wherein the parent structurecomprises a block of natural wood or a piece of paper formed ofcellulose fibers.

38. The method of any of clauses 25-37, further comprising, prior to(a), forming the elementary nanofibrils into a sheet, membrane, orpaper.

39. The method of any of clauses 25-38, further comprising, after (d),disposing the solid-state ion conducting structure between a pair ofelectrodes to form a battery, the metal-fibril complex with intercalatedsecond ions acting as a solid-state separator for the battery.

40. The method of any of clauses 25-39, further comprising, after (d),interspersing the metal-fibril complex with intercalated second ionswithin an electrode of a battery to act as an ion-conductive additivefor the electrode.

41. The method of any of clauses 25-40, further comprising, after (d),using the metal-fibril complex with intercalated second ions as asolid-state ion-conducting component in a device.

42. The method of any of clauses 25-41, wherein the device is a battery,a fuel cell, a supercapacitor, a transistor, a thermal power harvestingdevice, an electricity generating device, or an ion separating device.

43. An ion-conducting structure, comprising: a metal-fibril complexformed by one or more elementary nanofibrils, each elementary nanofibrilbeing composed of a plurality of polymer molecular chains withfunctional groups, each elementary nanofibril having a plurality ofmetal ions, each metal ion acting as a coordination center between thefunctional groups of adjacent molecular chains so as to form arespective ion transport channel between the molecular chains.

44. The ion-conducting structure of clause 43, wherein the metal-fibrilcomplex comprises a plurality of second ions, each second ion beingdisposed within a respective one of the ion transport channels so as tobe intercalated between the corresponding molecular chains.

45. The ion-conducting structure of any of clauses 43-44, wherein themetal-fibril complex is a solid-state structure.

46. The ion-conducting structure of any of clauses 43-45, wherein themetal-fibril complex comprises polysaccharide, poly(vinyl chloride)(PVC), poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA),poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(ethylmethacrylate) (PEMA), poly(methyl methacrylate) (PMMA), poly(ethyleneterephthalate) (PET), polyethylene (PE), poly(ethylene naphthalate)(PEN), polyamide (PA), poly(vinylidene chloride) (PVDC), polylactic acid(PLA), or combinations thereof.

47. The ion-conducting structure of any of clauses 43-46, wherein eachpolymer molecular chain comprises a naturally-occurring polysaccharide.

48. The ion-conducting structure of any of clauses 43-47, wherein thenaturally-occurring polysaccharide comprises cellulose, chitosan,chitin, or combinations thereof.

49. The ion-conducting structure of any of clauses 43-48, wherein theplurality of metal ions comprises copper (Cu), zinc (Zn), aluminum (Al),calcium (Ca), iron (Fe), or combinations thereof.

50. The ion-conducting structure of any of clauses 43-49, wherein theplurality of second ions comprises lithium (Li+), sodium (Na+),potassium (K+), magnesium (Mg+), protons (H+), or combinations thereof.

51. The ion-conducting structure of any of clauses 43-50, wherein awidth of each ion transport channel is less than 2 nm.

52. The ion-conducting structure of any of clauses 43-51, wherein themetal-fibril complex has a plurality of the elementary nanofibrils andis formed as a sheet, film, or membrane.

53. The ion-conducting structure of any of clauses 43-52, wherein themetal-fibril complex has a conductivity of at least 10⁻⁴ S/cm.

54. The ion-conducting structure of any of clauses 43-53, wherein acontent of total liquid within the metal-fibril complex is less than orequal to 10 wt %.

55. The ion-conducting structure of any of clauses 43-54, wherein acontent of bound liquid within the metal-fibril complex is less than orequal to 8 wt %.

56. A battery comprising: first and second electrodes, one of the firstand second electrodes operating as a cathode and the other of the firstand second electrodes operating as an anode; and a solid electrolytemembrane between the first and second electrodes, wherein the firstelectrode, the second electrode, the solid electrolyte membrane, or anycombination thereof comprises the ion-conducting structure of any ofclauses 43-55.

57. The battery of clause 56, wherein the anode comprises graphite,silicon, carbon, or combinations thereof.

58. The battery of any of clauses 56-57, wherein the cathode compriseslithium cobalt oxide (LCO) (LiCoO₂), lithium manganese oxide (LMO)(LiMn₂O₄), lithium iron phosphate (LFP) (LiFePO₄/C), lithium nickelcobalt manganese oxide (NMC) (LiNiCoMnO₂), lithium nickel manganesespinel (LNMO) (LiNi_(0.5)Mn_(1.5)O₄), lithium nickel cobalt aluminumoxide (NCA) (LiNiCoAlO₂), sulfur-carbon (S/C) composite, or combinationsthereof.

59. A fuel cell comprising: first and second electrodes, one of thefirst and second electrodes operating as a cathode and the other of thefirst and second electrodes operating as an anode; and a proton exchangemembrane between the first and second electrodes, wherein the firstelectrode, the second electrode, the proton exchange membrane, or anycombination thereof comprises the ion-conducting structure of any ofclauses 43-55.

60. The fuel cell of clause 59, wherein the anode, the cathode, or boththe anode and cathode comprise a metal, graphite, carbon composite,carbon polymer composite, or combinations thereof.

61. A method, comprising: (a) forming a metal-fibril complex byimmersing a plurality of elementary nanofibrils within an alkalinesolution and a plurality of metal ions dissolved therein, eachelementary nanofibril being composed of a plurality of polymer molecularchains with functional groups, the immersing being such that hydrogenbonds between adjacent functional groups of the polymer molecular chainsare broken so as to expose the functional groups and such that thedissolved metal ions from the alkaline solution form coordination bondswith the exposed functional groups.

62. The method of clause 61, further comprising: (b) intercalatingsecond ions between adjacent molecular chains of the metal-fibrilcomplex by immersing the metal-fibril complex in a first solution havinga plurality of the second ions dissolved therein.

63. The method of any of clauses 61-62, wherein the alkaline solutionhas a concentration of at least 5% (w/v).

64. The method of any of clauses 61-63, wherein the alkaline solutionhas a concentration of at least 20% (w/v).

65. The method of any of clauses 61-64, further comprising: (c)replacing free liquid in the metal-fibril complex by immersing themetal-fibril complex in an organic solvent.

66. The method of any of clauses 61-65, further comprising: (d) dryingthe metal-fibril complex such that a total content of liquid within themetal-fibril complex is less than 10 wt %, thereby forming themetal-fibril complex with intercalated second ions as a solid-state ionconducting structure.

67. The method of any of clauses 61-66, wherein the first solution isthe organic solvent, and the intercalating of (b) and the replacing freeliquid of (c) are performed simultaneously.

68. The method of any of clauses 61-67, wherein the first solution isseparate from the organic solvent, and the intercalating of (b) isperformed before or after the replacing free liquid of (c).

69. The method of any of clauses 61-68, wherein the drying in (d) issuch that a content of bound liquid within the metal-fibril complex isless than or equal to 8 wt %.

70. The method of any of clauses 61-69, wherein the metal-fibril complexcomprises polysaccharide, poly(vinyl chloride) (PVC), poly(vinylalcohol) (PVA), poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), poly(ethyl methacrylate) (PEMA), poly(methylmethacrylate) (PMMA), poly(ethylene terephthalate) (PET), polyethylene(PE), poly(ethylene naphthalate) (PEN), polyamide (PA), poly(vinylidenechloride) (PVDC), polylactic acid (PLA), or combinations thereof.

71. The method of any of clauses 61-70, wherein the plurality of metalions comprises copper (Cu), zinc (Zn), aluminum (Al), calcium (Ca), iron(Fe), or combinations thereof.

72. The method of any of clauses 61-71, wherein the plurality of secondions comprises lithium (Li), sodium (Na), potassium (K), magnesium (Mg),or combinations thereof.

73. The method of any of clauses 61-72, wherein the alkaline solutioncomprises sodium hydroxide (NaOH), potassium hydroxide (KOH), lithiumhydroxide (LiOH), or combinations thereof.

74. The method of any of clauses 61-73, wherein the first solutioncomprises propylene carbonate (PC), ethylene carbonate (EC), dimethylcarbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC),or combinations thereof.

75. The method of any of clauses 61-74, wherein the organic solventcomprises dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylenecarbonate (PC), acetone, ethylene glycol diglycidyl ether (EGDGE), orcombinations thereof.

76. The method of any of clauses 61-74, wherein the organic solventcomprises DMSO, EGDGE, or combinations thereof, and after (c) thepolymer molecular chains have a crystalline morphology.

77. The method of any of clauses 61-74, wherein the organic solventcomprises DMF, PC, acetone, or combinations thereof, and after (c) thepolymer molecular chains have an amorphous morphology.

78. The method of any of clauses 61-77, further comprising, prior to(a), subjecting a parent structure containing the elementary nanofibrilsto a mechanical fibrillation process, a chemical fibrillation process,an enzymatic fibrillation process, or combinations thereof, so as toexpose the nanofibrils from the parent structure.

79. The method of any of clauses 61-78, further comprising, prior to(a), forming the elementary nanofibrils into a sheet, membrane, orpaper.

80. The method of any of clauses 66-79, further comprising, after (d),using the metal-fibril complex as an ion-conducting component.

81. The method of any of clauses 61-80, wherein the ion-conductingcomponent is part of a battery, a fuel cell, a supercapacitor, an ionselective membrane, or a power generation device.

82. A method comprising conducting ions using one or more elementarynanofibrils, each elementary nanofibril being composed of a plurality ofpolymer molecular chains with functional groups that have beenchemically-modified.

83. The method of clause 82, wherein the elementary nanofibrils comprisepolysaccharide, poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA),poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), poly(ethyl methacrylate) (PEMA), poly(methylmethacrylate) (PMMA), poly(ethylene terephthalate) (PET), polyethylene(PE), poly(ethylene naphthalate) (PEN), polyamide (PA), poly(vinylidenechloride) (PVDC), polylactic acid (PLA), or combinations thereof.

84. The method of any of clauses 82-83, wherein each polymer molecularchain comprises a naturally-occurring polysaccharide.

85. The method of any of clauses 82-84, wherein the naturally-occurringpolysaccharide comprises cellulose, chitosan, chitin, or combinationsthereof.

86. The method of any of clauses 82-85, wherein each elementarynanofibril comprises a plurality of metal ions, each metal ion acting asa coordination center between the functional groups of adjacent polymermolecular chains so as to form a respective ion transport channelbetween the molecular chains.

87. The method of any of clauses 82-86, wherein the plurality of metalions comprises copper (Cu), zinc (Zn), aluminum (Al), calcium (Ca), iron(Fe), or combinations thereof.

88. The method of any of clauses 82-87, wherein each elementarynanofibril further comprises a plurality of second ions, each second ionbeing disposed within a respective one of the ion transport channels soas to be intercalated between the corresponding polymer molecularchains.

89. The method of any of clauses 82-88, wherein the plurality of secondions comprises lithium (Li+), sodium (Na+), potassium (K+), magnesium(Mg+), protons (H+), or combinations thereof.

90. The method of any of clauses 82-89, wherein the functional groupshave been chemically modified by converting hydroxyl groups thereof tocarboxyl groups using a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)treatment.

91. The method of any of clauses 82-89, wherein the functional groupshave been chemically modified by etherification using a3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHPTAC) treatment.

92. The method of any of clauses 82-91, wherein the conducting ions bythe one or more elementary nanofibrils is part of: a discharging orcharging process of a battery or a supercapacitor; or an electricitygeneration process of a thermal power harvesting device, an osmoticpower generation device, or a fuel cell; or an electricity transmissionor modulation process of a transistor; or an ion transport process of anion-selective membrane.

Examples Example 1—Cu-Paper Solid-State Ion Conductor

Nanofibril cellulose dispersed in water was vacuum filtered forming ananocellulose membrane, which was further hot pressed to a nanocellulosepaper. The nanofibril cellulose paper was immersed in LiOH solution(saturated) with Cu wires for over one week until the paper turned blue.The blue nanocellulose paper was further immersed in DMF solutionfollowed by vacuum drying, repeated three times, to ensure dehydration.The DMF solvent was finally evaporated, obtaining dry nanocellulosepaper. The dry nanocellulose was soaked in Li-ion electrolyte (1 M LiPF₆dissolved in EC/DMC) in an argon-filled glove box. After drying toremove the EC/DMC solvent, the nanocellulose became a solid-state Li-ionconductor.

Commercial copy paper with randomly distributed cellulose was immersedin NaOH solution (20%) with Cu wires for over one week until the paperturned blue. The blue Cu-paper was further immersed in DMF solutionfollowed by vacuum drying, repeated three times, to ensure dehydration.The DMF solvent was finally evaporated, obtaining dry Cu-paper paper,which was ˜400 μm thick. The Cu-paper was further treated in Li-ionelectrolyte, 1 M LiPF₆ dissolved in EC/DMC (1/1, v/v), to obtainCu-paper solid-state ion conductor. The Cu-paper solid-state ionconductor had a structure similar to sheet 400 in FIG. 4A.

The ionic conductivity and other electrochemical properties can bemeasured by using the Cu-paper ion conductor with two Li electrodes onopposite sides of the Cu-paper. The resistance response againsttemperature of the Cu-paper electrolyte was measured by EIS at differenttemperature, as shown in FIG. 17. The ionic conductivity at roomtemperature (25° C.) was 0.34 mS/cm.

Example 2—Cu-Wood Solid-State Ion Conductor

Delignified, densified wood (e.g., white wood) was first treated by Cuand NaOH solution (aqueous) to form Cu-cellulose complexes. The surfacewater of the Cu-cellulose was gently wiped before immersing theCu-cellulose into DMF solution. The Cu-wood was soaked in DMF for 12hours and then dried in vacuum at room temperature. This process wasrepeated three times to ensure free water extraction from theCu-cellulose. After evaporating any free water and DMF solvent, a dryblue Cu-cellulose complex was obtained. The dry blue Cu-cellulosecomplex was transferred into an argon-filled glovebox and immersed in aLi-ion electrolyte, 1 M LiPF₆ dissolved in EC/DMC (1/1, v/v). Afterimmersion, the previously blue Cu-cellulose had turned green. TheCu-cellulose complex was then removed from solution and the solvents(EC/DMC) therein were evaporated in a glovebox and in vacuum to yield asolid-state Cu-cellulose ion conductor. The Cu-cellulose ion conductorhad a structure similar to membrane 410 in FIG. 4B.

The high ionic conductivity of the Cu-cellulose ion conductor wasmeasured by placing Li metal foils on opposite ends of the Cu-cellulosestrip, which measured 1.26 cm wide (W) and 0.665 mm thick (D).Resistances of the Cu-cellulose solid-state ion conductor were measuredwith different lengths (L) by EIS. The results are shown in FIGS.18A-18B. The resistances (R) shows a linear relation with the length L,in accordance with the Law of Resistance R=ρL/S. Hence, the Li-ionconductivity σ is calculated by

$\sigma = {\frac{1}{\rho} = {\frac{L}{R \cdot {WD}} = {5.3\mspace{14mu} {{mS}/{{cm}.}}}}}$

The ionic conductivity was orders of magnitudes higher that most solidpolymer electrolytes. For example, PEO-based polymer electrolytes haveionic conductivity of 10⁻⁶-10⁻⁸ S/cm.

The solid-state nature of the Cu-cellulose ion conductor wasdemonstrated by TGA (FIG. 18C) and tensile strength (FIG. 18D). TheCu-cellulose solid-state ion conductor (curve 1804) showed approximatelythe same weight loss as the white wood (curve 1802) below 100° C.,indicating minimal amount of liquid (e.g., free water, DMF, EC, DMC,etc.) in the solid-state Cu-cellulose ion conductor. Due to thecoordination of Cu²⁺ in the cellulose molecules, the Cu-cellulose solidion conductor (curve 1806) showed much enhanced tensile strength, ˜30MPa, much higher than the white wood (curve 1802) with the sametreatment process but without Cu and much higher than most solid polymerelectrolytes and gel polymer electrolytes. The robust mechanicalstrength demonstrates the merit of the solid electrolyte.

Dry Cu-cellulose was fabricated via the same aqueous intercalation andDMF replacement procedure as described above. The dry Cu-cellulose wasthen soaked in a Na-ion electrolyte, 1 M NaPF₆ dissolved in EC/DEC/DMC(1/1/1, v/v/v), followed by solvent evaporating in the glovebox to forma structure similar to membrane 410 of FIG. 4B. The Na-ion conductivitywas measured by the same method as described above, except that two Nametal foils were used in place of Li metal. The results are shown inFIG. 18E. The Na-ion conductivity of the Cu-cellulose solid ionconductor was 3.3×10⁻⁴ S/cm.

Example 3—Aqueous Batteries Employing Cu-Cellulose Aqueous IonConductors

Intercalation of ions, including Na⁺, K⁺, was observed within aCu-cellulose framework. The electrostatic field adjacent to the chargednanochannel walls typically redistributes ions while the mobility staysconstant. The electrical-double-layer-regulated ion movement is thusintrinsically limited to low electrolyte concentrations and cannotexceed the value for the bulk electrolyte under higher concentrations.However, as a molecular level building block with a hierarchicalstacking, the cellulose can be tuned at the molecular scale.Sub-nanometer channels were observed among the cellulose molecularchains where the confinement of solvated ions has been reduced to lessthan 1 nm. New transport phenomenon occurs within the sub-nm channels,where mobile ions are regulated by the charged walls and the confinedspacing. Moreover, improved mechanical properties can be obtained by themetal coordination between the cellulose molecular chains. For example,FIG. 19 compares the stress-strain curves for Cu-cellulose II metal ioncomplex (curve 1902) with aqueous cellulose II (curve 1904).

Molecular level interactions between ions and water molecules within theangstrom spacing were evaluated by a combination of conductivitymeasurement (FIGS. 20A-20B), transfer number and activation energymeasurements. The negatively charged molecular chains draw cations tothe channels while the Cu(OH)₆ ²⁻ groups within the channel furtherimpede the transport of anions via repulsive coulomb force. Theconductivity of bulk NaOH solution with concentrations from 10% up to40% w/w starts to show saturation around 20% and then decreases due tothe much-reduced ion mobility upon higher chance of ion collision. Foreach concentration, a Cu-cellulose film was immersed, and theconductivity was measured. An enhancement was observed at allconcentration levels. FIG. 20C shows the mobility enhancement due to thereduced hydration number of sodium and potassium.

The energy output of aqueous batteries is largely limited by the narrowvoltage window of their electrolytes. Aqueous Li-ion batteries offeradvantages in terms of safety, toxicity and cost over their non-aqueouscounterparts. However, the electrochemical stability window of aqueouselectrolytes is much narrower than that of the non-aqueous ones. Thisstability window is determined by the reductive and oxidative reactionsof salts, solvents or additive components in the electrolyte. The stableoperating voltage window of water is only 1.23 V, beyond which undesiredwater hydrolysis occurs. Expanding the operating voltage windowtherefore represents the core challenge in the development of practicalaqueous batteries.

A Na aqueous battery and Li aqueous battery were demonstrated with muchwidened electrochemical window with the aid of cellulose framework. Eachbattery had a structure similar to that of battery 500 in FIG. 5A (butusing an aqueous separator membrane) or that of battery 900 in FIG. 9(but without metal-fibril complex in the electrodes 904, 908). As shownin FIG. 21, the measured electrochemical window of aqueous NaCl inCu-cellulose framework (compare curve 2102 versus curve 2104) provides amuch-widened window of ˜2.4V. The loosely bond water molecule shows amuch-reduced activity in hydrolysis. With salt including but not limitedto LiCl, LiOH, NaCl, NaOH, et al, the window can expand to >2 V. Theconcentration of the salt can be largely tunable. The molecular scaleion channels provided by the cellulose effectively reduces the hydrationnumber of the ions, including, but not limited to Li+, Na+ and K+, whichincreases the cathodic limit and at the same time provide passivation ofthe negative electrode surface through the formation of the ions withsmaller hydration shell at the interphase. The absence of the free watermolecule suppressed the water hydrolysis including hydrogen and oxygenevolution at the electrodes. In addition, the ions with fewer hydrationnumber exhibits a higher conductivity which is beneficial for efficiention transport in battery components including separation membranes,cathode and anode.

Example 4—Thermoelectric Device Employing Ion-Conducting CelluloseMembrane

A membrane for selective ion diffusion (that is, the ability of thecellulose membrane to selectivity impregnate Na+ ions and repel OH− ionsof the NaOH solution that is in equilibrium with the cellulose membrane)was composed of well-aligned cellulose nanofibrils and is fabricated bya scalable method that involves cutting natural wood perpendicular tothe fiber growth direction followed by a delignification process thatinvolves using high concentration NaOH. The formation of cellulose II inthe resulting membrane leads to Na-cellulose complex formation afterelectrolyte infiltration. Oxidation of2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) enhances the negativecharge density of the cellulose nanofibrils, which leads to additionalenhancement in the thermally generated voltage (up to 24 mV K−1).

FIG. 22A shows the charge density of the natural wood, cellulosicmembrane and TEMPO-oxidized cellulosic membrane. FIG. 22B shows theconductance of the cellulosic membrane measured at different NaOHconcentrations. FIG. 22C shows measured differential thermal voltage ofvarious solutions and wood-based structures, in particular: aqueous NaOHsolution, polymer electrolyte (NaOH+PEO+deionized water), polymerelectrolyte infiltrated into the natural wood, the randomized cellulosicfibers, the cellulosic membrane and the oxidized cellulosic membrane(poly(ethylene oxide), PEO). The differential thermal voltage increasesfrom 6.5 to 10 mV K⁻¹ after adding PEO to the bulk NaOH solution. Thevalue increases further from 18 to 24 mV K⁻¹ for the devices composed ofaligned and oxidized delignified wood, respectively.

To produce a cellulose-based membrane, a chemical process was used toextract the lignin and the hemicellulose from the natural wood. Afterthis treatment, the naturally aligned cellulose nanofibers were theremaining component of the wood structure, which feature a negativelycharged surface that can be enhanced by TEMPO oxidation. An ionicthermoelectric device was constructed by infiltrating this cellulosicmembrane with NaOH-based polymer electrolyte and applying a temperaturedifference across the membrane. In this manner, a Seebeck coefficientwas achieved that exceeded that of the bulk electrolyte. The enhancedSeebeck coefficient originates from the ionic selectivity of thenegatively-charged cellulose nanofibers and the resulting development ofsurface-charge-governed ion transport, where a natural asymmetry interms of the number density of positive and negative ions occurs withinthe nanochannels. This is a unique ionic thermoelectric device that useschemically-delignified wood (i.e., cellulosic membrane). The proposedcellulosic membrane relies on nanoscale confinement of the oxidized,aligned cellulose molecular chains to promote ionic selectivity, whichenhances the overall thermoelectric performance Based on a combinationof microscopy, neutron scattering, and synchrotron X-ray diffractiontechniques, a multiscale aligned structure was determined withhierarchical spacing of 30 nm, 2 nm, and 0.7 nm between cellulose fiberbundles, elementary fibers, and molecular chains, respectively. Afterinfiltrating electrolyte into the cellulosic membrane and subjecting themembrane to an axial temperature gradient, the ionic thermoelectricdevice exhibited a Seebeck coefficient of 24 mV/K, which is more than atwo-fold improvement over the highest value reported to date.Consequently, a high power factor of 1150 μW/m·K² was demonstrated atroom temperature. The enhanced ionic selectivity can be attributed toeffective sodium ion insertion into the charged molecular chains of thetype II cellulosic membrane, which does not occur in natural wood ortype I cellulose.

With this material a flexible and biocompatible ionic thermoelectricdevice can be manufactured at large-scale with potential in a range ofapplications, such as low-grade waste heat recovery (e.g., thermal powerharvesting device) and skin electronics. This approach demonstrates theuse of nanoscale engineering to improve ionic thermoelectric performancewhile utilizing sustainable materials. A significant enhancement in thethermally-generated voltage was demonstrated after infiltratingelectrolyte into a cellulosic membrane due to enhanced ionic selectivitywithin the charged molecular chains in conjunction with the synergisticSoret effect. The ion selective membrane was composed of well-alignedcellulose nanofibers and was fabricated by a scalable method thatinvolves cutting natural wood perpendicular to the fiber growthdirection followed by a delignification process using 10% wt NaOHsolution. The resulting membrane was composed of type II cellulose,which enables Na-cellulose complex formation. TEMPO oxidation furtherenhanced the negative surface charge density of the cellulosenanofibers, which leads to additional enhancement in the Seebeckcoefficient (up to 24 mV/K) and can be used for a wide range ofapplications, including temperature sensing and low-grade thermal energyharvesting.

The fabricated structures inherit the vertically aligned nature of thewood channels (e.g., lumen), which are themselves composed of verticallyaligned cellulose nanofibers. After delignification, the wood sample wascomposed of ˜85% cellulose fibers with a negligible amount ofhemicellulose and lignin. Using SANS, the diameter of the elementaryfibrils of the membrane was found to be ˜4 nm. When immersed inelectrolyte, a Na-cellulose complex was observed and a spacing of ˜0.6nm among the molecular chains was determined from the synchrotron XRDspectra. Thermogravimetric analysis at a heating rate of 10° C./minindicated that the cellulosic membrane was thermally stable up to 315°C.

In the fabricated thermoelectric device, selective ion transport occursin the cellulosic membrane when the two electrode contacts are exposedto different temperatures. The thermoelectric device had a structuresimilar to that of device 1200 in FIG. 12. The charge density is a keyfactor in determining the nanofluidic conductivity. The charge densitiesof the natural wood, cellulosic membrane and oxidized cellulosicmembrane were determined from conductometric titration as 0.196±0.008,0.157±0.007 and 0.246±0.013 mmol g⁻¹, respectively (FIG. 22A). Thecharge density of the cellulosic membrane was lower than that of thenatural wood as hemicellulose is partially removed by the alkalinesulfite treatment, leading to reduced glucuronic acid content. Afteroxidation, the charge density increased to 0.246 mmol/g as the primaryhydroxyl groups on cellulose are oxidized to carboxylic groups, whichcan dissociate into negatively charged carboxylate groups more easily,thereby enabling a higher charge density. This electrostatic fieldsurrounding the cellulose nanofibrils enables surface-charge-governedion transport along the fiber direction by providing a desirabledisparity between the concentrations of Na+ and OH− ions inside thisnanospace. This disparity facilitates the transport of Na+ ions andimpedes the directed movement of OH− ions.

The ionic conductivity of the NaOH solution after infiltration into thecellulosic membrane is shown in FIG. 22B. The minimum concentration ofmobile ions was determined by the fixed charge on the cellulose,providing a lower limit of conductivity. Hence, at such small NaOHconcentrations, the conductivity is virtually constant (and entirelydictated by the fixed charge on the cellulose), as shown in the plateauof the conductivity versus salt concentration dependence in curve 2202.On the other hand, at larger NaOH concentrations, that is, atconcentrations that exceed the concentration of the fixed charges of thecellulose, the conductivity increases with increasing bulk NaOHconcentration (that is, the concentration of the NaOH solution that isin equilibrium with the cellulose membrane with Na+ ions impregnatinginto the cellulose membrane). Hence in this conductivity versusconcentration plot, the plateau region is distinguished from the linearregime by the ratio of the fixed charges of the cellulose to the bulkNaOH concentration (curve 2204). The conductance of the cellulosicmembrane thus becomes slightly lower than that of the NaOH bulksolution, which may be due to the reduction of the equivalent area forthe ion transport when infiltrated into the cellulosic membrane.However, when the NaOH concentration increased to 2.5 wt % (0.625 moll-1), the infiltrated cellulosic membrane showed an even higher ionicconductivity. This is due to the type of cellulose that exists in themembrane (that is, Cellulose II), where Na ion insertion occurs andallows the previously unused 0.6 nm channels to contribute to the iontransport.

The cellulosic devices were heated uniformly to study the temperaturedependence of ionic conductivity. An ionic conductivity of ˜20 mS cm-1was measured with the polymer electrolyte consisting of PEO, deionizedwater and NaOH infiltrating cellulose membrane under room temperature.An increasing trend can be observed between 20° and 60° C. The ionicdifferential thermal voltage of various cellulosic membranes infiltratedwith different solutions were then compared, as shown in FIG. 22C.

Thus, a binary ionic system without intermolecular reactions has anionic differential thermal voltage that is proportional to thedifference between the thermophoretic mobilities for the positive andnegative ions (μ⁺-μ⁻). To measure the ionic differential thermalvoltage, the same polymer electrolyte consisting of PEO, deionized waterand 0.625 mol NaOH was prepared and infiltrated into various cellulosicmembranes between two platinum electrodes, in which the temperature ofthe hot and cold sides were simultaneously monitored. The absence of theinterface reaction causes the ions to accumulate at the platinumelectrodes, which allowed us to evaluate the differential thermalvoltage generated by ion redistribution in the electrolyte. The bulkelectrolyte exhibits a value of 10 mV K⁻¹, which is in good agreementwith a similar bulk electrolyte.

To understand the structure-property relationships, evaluation of thethermally generated voltage was performed, and comprehensive structuralanalysis was conducted of different types of membrane, including naturalwood, the delignified membrane featuring pure cellulose I polymorph andthe delignified membrane that consisted of cellulose II polymorph.Cellulose I, as found in natural wood and H₂O₂-delignified cellulosicmaterial, cannot facilitate Na+ ion insertion after infiltration withthe alkaline electrolyte. A differential thermal voltage of 11 mV K⁻¹and 12 mV K⁻¹ was obtained for the natural wood and type I cellulosicmembranes, respectively. The voltage generated with the formation of theNa-cellulose complex (with the membrane delignified by NaOH/Na₂SO₃/H₂O₂)exhibits a largely enhanced signal: 19±1 mV K⁻¹, as shown in FIG. 22C.

The aligned structure of the cellulosic membrane is also beneficial tothe ionic differential thermal voltage. The membranes featuring randomlydistributed and orthogonally oriented cellulose fibers exhibitcomparable yet lower values than the proposed cellulosic device.Moreover, the large pores (that is, lumens) in the membrane do notcontribute to the overall thermally generated voltage since bothdensified and epoxy-filled membranes showed negligible performancechange. In addition, cellulosic membranes made from different parts ofwood (that is, heartwood and sapwood) exhibit similar performances.

To further identify the effect of charge density on ion transport andselectivity, the aligned cellulosic membrane was oxidized to increasethe amount of surface charge. Subsequently, after infiltration withelectrolyte, an ionic differential thermal voltage of 24 mV K⁻¹ wasobtained as shown in FIG. 22C. The high performance arises from theenhanced interaction between the negatively charged nanofibrils and themobile ions in the electrolyte. The experimentally obtained power factor(1,150 μW m K⁻²) represents an order of magnitude enhancement comparedto previously reported data. The thermal conductivity of theelectrolyte-infiltrated cellulosic membrane was found to be as low as0.48±0.03 W m K⁻¹, which is beneficial for thermal energy conversion.The observed thermally generated voltage is due to selective iondiffusion driven by temperature-dependent electrophoretic ion movement,which is typically not considered a conventional thermoelectric exampleinvolving band structural engineering.

The thermal charging behavior of the ionic conductor was evaluated. Acellulosic membrane was infiltrated with electrolyte and sandwichedbetween two platinum electrodes for testing purposes. The sealed devicewas exposed to an external temperature difference of 5.5° C. Thecharging and discharging behaviors of the membrane are shown in FIGS.23A-23B, respectively. The behavior of the membrane (curves 2302, 2306)is compared against bulk electrolyte (curves 2304, 2308). Theelectrolyte-infiltrated membrane (2302) was charged to 0.118 V while itsbulk counterpart (2304) was charged to 0.050 V. Theelectrolyte-infiltrated cellulosic membrane (2302) was charged to 0.118V with a response time of ˜70 s while its bulk counterpart (2304) wascharged to 0.050 V with a much longer response time of ˜380 s. Note thatthe electrolyte-infiltrated membrane with the oxidized cellulose can becharged to more than twice that of the pure bulk polymer electrolyte dueto the presence of the nanofluidic channels. Furthermore, the muchshorter response time of the electrolyte-infiltrated membrane isattributed to the enhanced ionic conductivity in the aligned nanofluidicchannels.

FIG. 23B displays the discharging behavior of both theelectrolyte-infiltrated membrane (2306) and bulk electrolyte (2308),where the amount of charge generated under the applied temperaturedifference can be obtained by integrating the amount of current flowduring the discharging process. The times to completely discharge thenanofluidic system and bulk electrolyte device at 500 nA cm⁻² were 170 sand 4 s, respectively. The longer discharging duration for thecellulosic membrane is due to a larger amount of charge stored in thenanofluidic membrane under the same external thermal bias. For acontinuous operation, an oxidation-reduction can react on the electrodesurfaces or a mechanical system can be incorporated that provides anoscillating heat source.

Example 5—Ion Conducting Membrane Formed from Delignified Wood

A highly efficient and tunable ion regulation using a cellulose membranewas composed of aligned nanochannels. Cellulose nanofibers were exposedafter extraction of intertwined lignin and hemicellulose from thenatural wood. Due to the dissociation of the surface functional groups,the charged cellulose nanofiber surface can attract layers ofcounter-ions adjacent to the fibers, with an exponentially decaying ionconcentration toward the center of the channel. The interface-dominatedelectrostatic field surrounding the cellulose nanofibers providessurface-charge-governed ion transport along the fiber direction,enabling desirable ionic separation.

The surface charge and geometry of the nanochannels can be easily tunedto modify the ionic conductivity of the membrane. Owing to the abundanceof the functional groups on the cellulose nanofibers, the surface chargedensity can be tuned via chemical stimuli. A high surface charge densityof −5.7 mC·m⁻² was demonstrated after converting the hydroxyl groups tocarboxyl groups. Additionally, large tunability of the channel size wasattained, up to an order of magnitude. In this manner, a highsurface-charge-governed ionic conductivity of ˜2 mS·cm⁻² was observed ata KCl concentration of less than 10⁻² mol·L⁻¹.

The nanofluidic performance of the cellulose membrane was evaluatedusing an ionic conductivity setup. Carboxyl groups have a greatertendency to dissociate into negatively charged carboxylates compared tohydroxyl groups, leading to a higher charge density and therefore ahigher negative zeta potential in deionized water. Therefore, TEMPOoxidation was applied to the cellulose to convert the primary hydroxylgroups 2402 on the surface chains of the cellulose crystallites intocarboxyl groups 2404, as shown in FIG. 24B. The resulting oxidizedcellulose membrane exhibited a higher Zeta potential of −78 mV, comparedwith −45 mV for just delignified cellulose, as shown in FIG. 24A. Therespective ionic conductivity of these materials in KCl solutions areshown in FIG. 24C. The ion transport behavior in both cellulosemembranes exhibited a conductivity plateau orders of magnitude higherthan bulk solution for concentrations below ˜10⁻² mol·L⁻¹. Within thesurface-governed ion transport region, a conductivity as high as ˜2mS·cm⁻¹ was obtained for the oxidized membrane, compared with 1.1mS·cm⁻¹ for the unmodified counterpart, indicating the effectiveness ofmodifying the surface functional groups to tune the ion transportbehavior.

Using eq. (1), the surface charge of the membranes was estimated basedon the zeta potential:

σ=εε₀ζ/λ_(d),  (1)

in which σ is the surface charge, ε is the dielectric constant, ε₀ isthe permittivity of vacuum, ζ is the Debye length, and λ_(d) is the zetapotential, which was −3.2 mC·m⁻² and −5.7 mC·m⁻² for the as madecellulose and oxidized cellulose, respectively. With the estimate of thesurface charge for these samples, the overall conductivity trend can befitted using the following equation:

κ=Ze(μ₊+μ⁻)CN _(A)+2σμ₊ /h,  (2)

in which Z is the cation valence, μ₊ is the cation mobility, μ⁻ is theanion mobility, C is the ion concentration, N_(A) is Avogadro's number,and h is the channel diameter of the nanofluidic cellulose system. Asshown in FIG. 24C, the fit of eq. (2) agrees well with the experimentaldata of the ionic conductivity versus KCl concentration and allowscalculation of a channel diameter of ˜2 nm for both cellulose membranes.The difference in the value of the ionic conductivity plateaus for eachmembrane may be attributed to the difference of the surface chargedensities between the materials.

The effect of channel geometry on the ionic conductivity wasinvestigated by preparing an un-densified cellulose membrane andcomparing its performance with the densified sample, as shown in FIG.24D. The un-densified samples exhibited an ionic conductivity plateau of0.2 mS·cm⁻¹, one order lower than that of the densified sample. The fitof the un-densified cellulose membrane conductivity results indicated achannel diameter of around 20 nm, which is about 10 times larger thanthat of the densified cellulose membrane (channel diameter ˜2 nm).

Example 6—Ionic Transistor Employing Ion-Conducting Cellulose Membrane

To explore the use of this cellulose nanofiber membrane as an ionregulation device, the ionic rectification effect was demonstrated inthe material acting as a flexible transistor with electrical gating, inwhich the cellulose membrane can preferentially accumulate ions thathave the opposite charge as the channel walls. Silver paste was paintedon the membrane to act as the gating metal, and a 10⁻⁶ mol·L⁻¹ KClsolution was used as the liquid electrolyte. The ionic transistor devicehad a structure similar to transistor 1300 in FIG. 13.

The gating voltage was controlled by a Keithley 2400 power source whilethe ionic current-voltage characteristics were recorded. When the gatingvoltage was negative, the local concentration of K⁺ should furtherincrease under the gate, which will contribute a large cationic currentdensity. Meanwhile, positive gating will repel K⁺ and lead to an evenlower current density than the neutral gating condition. FIGS. 25A-25Bshow the ionic currents measured under different gating potentials from−2 V to 2 V (curves 2502-2508). The ion conductivity under Vg=−2 V wasabout one order of magnitude higher than the value under Vg=2 V, andequivalent to that of 10-2 mol·L⁻¹ KCl, indicating an efficientaccumulation of positive ions with negative gating. The device exhibitsa negligible electrical gate leakage current, which was measured to bebelow the noise floor of the Keithley 2400.

The cellulose membrane was flexible and even foldable. A ribbon of themembrane that can be twisted and wrapped around a finger. To observe howfolding affects the ionic conductivity performance, a membrane 2 cm×2mm×1 mm in size was used, and the current under an applied voltage of0.5 V was recorded as the membrane was folded. The ionic conductivityunder a concentration of 10⁻⁶ mol·L⁻¹ KCl exhibited minimal changes uponfolding, with no notable performance degradation for a folding angle ofup to 150°.

Example 7—Osmotic Power Generation Device Employing Ion-ConductingCellulose

Wood-based materials were utilized as functional nanofluidic structures.Epoxy was used to infiltrate into the large pores while leaving theion-selective nanopores open. When exposed to salinity difference, theelectrical double layer formed along the nanocellulose allows thecations to efficiently pass through while impeding the transport ofanions, establishing an electrical potential in an opposite direction asthe cation movement direction. Notably, the structure was robust notonly in axial direction but also in transverse direction. The numerousnanofluidic channels are well-integrated within the polymer infiltratedwood with enhanced stability in seawater.

A wood membrane was disposed between waters with different salinitylevel (e.g., seawater and freshwater) for energy generation. The setupwas similar to power generation system 1400 in FIG. 14. Nanocellulosefibers comprising wood channels exhibit a negative surface charge (−3mC/cm², 0.16 mmol/g) that can preferentially induce a net flow ofcations along the channels. Further chemical treatment can be undertakento further increase the charge density of the cellulose fibers, forexample, by conversion of hydroxyl group to carboxyl groups thatdissociate more easily in solution. Salinity difference provide thestreaming energy required to promote a continuous net ion flow. The netion flow through the wood channels then induces a measurable outputelectrical signal (voltage and current density). In this way, anelectric field is established across the wood membrane. FIGS. 26A-26Bshow the measured current-voltage characteristics and power output,respectively. A short circuit current density of 0.025 mA/cm² and anopen circuit voltage of 0.2 V was obtained between 100 m mol/L and 0.001m mol/L NaCl (e.g., curve 2604).

Example 8—Ion Conducting Membrane Formed from Densified Wood

A high surface charge density of −5.7 mC·m⁻² was demonstrated afterconverting hydroxyl groups to carboxyl groups. Additionally, a highsurface-charge-governed ionic conductivity of ˜2 mS·cm⁻² was observed ata KCl concentration of less than 10⁻² mol·L⁻¹. In the present example,CHPTAC was used as etherifying agent to modulate the surface charge ofnatural wood. In a mixed solution containing sodium hydroxide, urea, anddistilled water (7.5:11:81.5), epoxide was provided in situ from CHPTAC,and quaternized cellulose could be synthesized via the reaction betweenthe epoxide and cellulose sodium alkoxide. After chemical treatment withCHPTAC for ten hours, the natural wood was converted into quaternizedwood, which presents a positive charge in electrolyte solution. The mainreaction is the cationization reaction of cellulose, as well as thequaternized hemicelluloses due to the similar functional groups ascellulose.

Compared with the molecular structure of cellulose in natural wood,cationic functional groups, for example, —(CH₃)₃N⁺, in cationic woodmembrane bond via the extended side chain of cellulose. The molecularstructure of the resulting cationic wood nanocellulose is grafted by thequaternary ammonium groups —(CH₃)₃N⁺ in the main chain, rendering thewood nanochannels positively charged. In addition, significant hydrogenbonding occurs between the quaternary ammonium ions and the cellulosechains during a subsequent densification process, which contributes tothe high stability and mechanical strength of the resulting cationicwood membrane.

Main surface properties of the natural wood and cationic wood membranewere evaluated using ζ-potential measurements in high purity water at25° C. The ζ-potential measurement confirmed the charge reversal of thenatural wood after the chemical treatment, from −27.9±0.95 mV (fornatural wood) to +37.7±1.65 mV after the cationic modification, due tothe bonding of the quaternary ammonium salt group to the cellulose inthe cell walls. Cationic wood membrane from densification of quaternizedwood, they have the same zeta potential and there is no new chemicalgroup formed. To the best of our knowledge, this is the first report ofa positively charged wood by directly chemically modifying the celluloseof wood.

The nanofluidic performance of the cationic wood membrane wasinvestigate by an ionic conductivity setup. FIG. 27A shows a series ofNyquist plots that were measured for the cationic wood membrane atvarious KCl concentrations (10⁻⁶-10⁻¹ M, e.g., curves 2712-2702,respectively). Ionic conductivity of the bulk KCl solution 2720, naturalwood 2718, quaternized wood 2716, and cationic wood membrane 2714 werecalculated in FIG. 27B based on the following equation:

λ=l/SR

in which l, S, and R are the length, cross-sectional area, and themeasured resistance, respectively. The ionic conductance of naturalwood, quaternized wood, and cationic wood membrane deviated from thebulk behavior, which can be explained by the nanofluidic surfacecharge-governed transport mechanism. At low concentrations, theconductivity values of the natural wood and quaternized wood areconstant and independent of the bulk KCl concentration below 10⁻⁴ M(FIG. 27B).

The natural wood exhibited a conductivity of 4.5×10⁻⁵±2.1×10⁻⁶ S/cm in10⁻⁶ M KCl solution. The conductance of quaternized wood was1.2×10⁻⁴±9.5×10⁻⁶ S/cm in 10⁻⁶ M KCl solution, which is 2 times higherthan that of natural wood. Furthermore, a remarkable enhancement of theconductance to about 1.3×10⁻³±9.9×10⁻⁶ S/cm was observed in the cationicwood membrane, which is 25-times higher than the natural wood. In termsof the cationic wood-based nanofluidic device, the conductivity deviatedfrom the bulk behavior for KCl concentration below 0.1 M (FIG. 27B), andthe conductance exhibited a plateau with a small increase, confirmingthat the surface-governed ion transport in the smaller nanochannels ofthis material resulted in a lower limit of ionic conductivity.

At high ionic concentration, the conductance of the natural wood andquaternized wood were similar to the bulk KCl solution. Note that theionic conductivity calculated for the cationic wood membrane wasrelatively smaller than that of the bulk solution due to the smallerequivalent area of the ion transport after the structural densificationof the wood. As shown in FIG. 27C, the conductance of natural wood,quaternized wood, and cationic wood membrane in the concentration of10⁻⁴ M KCl is about 2 times, 9 times, and 90 times higher than bulksolution, respectively. Note that the ion conductance of the cationicwood membrane is higher than those fabricated by other layered materialsin KCl electrolyte solution, such as graphene oxide based nanofluidics,aligned mesoporous silica films, boron nitride layers, potentially dueto the high surface charge density and smaller channel diameters.

Example 9—Cationic Wood Membrane

A strong densified wood membrane with nanofluidic channels was directlymade from natural balsa wood via chemical modification anddensification. Etherification bonds the cationic functional group(—(CH₃)₃N⁺Cl⁻) to the cellulose backbone, converting negatively charged(ξ-potential of −27.9 mV) wood into positively charged wood (+37.7 mV).Densification eliminates the large pores of the natural wood, leading toa highly laminated structure with the oriented cellulose nanofiber and ahigh mechanical tensile strength of ˜350 MPa in dry condition (20 timeshigher than the untreated counterpart) and ˜98 MPa in wet condition(×5.5 increase compared to the untreated counterpart). The nanoscalegaps between the cellulose nanofibers act as narrow nanochannels withdiameters smaller than the Debye length, which facilitates rapid iontransport that is 25-times higher than the ion conductance of thenatural wood at a low KCl concentration of 10 mM. The fabricatedcationic wood membrane exhibits an enhanced mechanical strength andexcellent nanofluidic ion-transport properties, representing a promisingdirection for developing high performance nanofluidic material fromrenewable, and abundant nature-based materials.

To further investigate the ion selectivity behavior of the natural woodand cationic wood membrane, current-voltage curves were measured for iontransport through natural wood and cationic wood membrane undertransmembrane concentration difference of salt concentration gradients(C_(high)/C_(low)=100 and 1000-fold). The electrolyte concentration atone side of wood membrane (or natural wood) was fixed at 10 mM(1000-fold) or 1 mM (100-fold) KCl, while the concentration at the otherside was 0.01 mM KCl. The setup was similar to the power generationsystem 1400 in FIG. 14. The direction of the short-circuit current is inaccordance with the net flow of the charged ions from high concentrationto low concentration, which can serve as an indication of the selectiveions diffusion of the channels.

In order to test the selectivity of the natural balsa wood,polydimethylsiloxane (PDMS) was used to seal the large channels in woodmesoporous structure. The short-circuit current (I_(SC)) and membranepotential can be read from the intercepts on the current and voltageaxes. As is shown in FIG. 28A, both K⁺ and Cl⁻ ions diffuse from highconcentration (10 mM) to low concentration (0.01 mM KCl) without anexternal voltage (V=0), and a net current (I_(SC)) could be observedupon the selective ion diffusion of Cl⁻ over K⁺ in cationic woodmembrane. The same result was obtained from a high concentration of 1 mMKCl to a low concentration of 0.01 mM KCl (C_(high)/C_(low)=100). Due tothe opposite charge selectivity, the short-circuit current and membranepotential of the natural wood is of opposite polarity to the cationicwood membrane. FIG. 28A further indicates that the natural wood iscation selective and the cationic wood membrane is anion selective. Thecationic wood membrane shows the same selectivity for negative ions asthe quaternized wood due to their similarity of surface charge type. Thecharge selectivity of the natural wood and quaternized wood wasconfirmed by energy-dispersive spectroscopy (EDS). The natural wood andquaternized wood were immersed in 10⁻¹ M KCl for 24 h, followed bywashing with high purity water to remove unbound ions. As shown in FIG.28B, the EDS spectrum shows the distribution of Cl⁻ and K⁺ in thecrossing section of quaternized wood, suggesting that there were moreCl⁻ ions than K⁺ ions. In comparison, similar distribution of Cl⁻ and K⁺in the crossing section of natural wood without infiltration of PDMSsuggests a low ion selectivity. Based on these results, calculated themolar ratio of Cl⁻ ions and K⁺ ions in quaternized wood was calculatedto be approximately 4.1:1, which further supports the anionicselectivity of the quaternized wood. The overwhelming content of Cl⁻over K⁺ in quaternized wood also verifies the negative charged ionselectivity.

Example 10—Fuel Cell Device Employing Ion-Conducting Chitosan

As shown in FIG. 29A, chitosan 290 has a molecular chain formed by 1,4linked-2-deoxy-2-aminoglucose, which is generated from the deacetylationreaction of chitin. It shows many interesting properties, such as beingnon-toxic, biodegradable, and biocompatible. Chitosan has several polarfunctional groups, such as hydroxyl and amino groups 2902, which can actas donors and form complexes with inorganic materials. In a fabricatedexample, a chitosan film was obtained from cast-drying ofchitosan/acetic acid solution. The film was then immersed in Cu and NaOHsolution (aqueous) to form chitosan-Cu nanostructures, for example, asshown in FIG. 29B. The Cu metal 2904 acts a coordination center in thecoordination bond to the surrounding functional groups 2902.

Proton donors were introduced into the chitosan-Cu film by immersion inappropriate solution, for example, by one or more of ammonia, ammoniumnitrate, ammonium chloride, ammonium sulfate, polyacrylic acid, citricacid, or the like. Finally, a chitosan-Cu as a proton conductor wasfabricated after drying in oven. In embodiments, such drying can includevacuum drying, freeze drying, and/or critical point drying. TheCu-chitosan film can be used as solid electrolyte in a fuel cell, forexample, as PEM 604 in the fuel cell system 600 of FIG. 6A.

The high proton conductivity of the chitosan-Cu was measured by placingtwo steel metal foils on the two ends of a chitosan-Cu stripe in atemperature and humidity-controlled box. Resistances of the chitosan-Cuproton conductor were measured by EIS. The proton conductivity σ iscalculated by

$\sigma = {\frac{1}{\rho} = {\frac{L}{R \cdot {WD}} = {55\mspace{14mu} {{mS}/{{cm}.}}}}}$

As shown in FIG. 29C, the proton conductivity of chitosan-Cu issignificantly higher than other polymers (PS), natural polymers(cellulose or pure chitosan) and MOF (UIO-66). For example, the protonconductivity of the chitosan-Cu conductor is almost the same ascommercially-available electrolytes for fuel cells (such as Nafion).However, chitosan-Cu has remarkably low methanol permeability of10⁻⁷˜10⁻⁸ cm² s⁻¹, which is much lower than Nafion (3*10⁻⁶ cm²s⁻¹). Thelow methanol permeability of chitosan-Cu displays extremely lowerdiffusion of methanol from anode to cathode in fuel cell and can improvethe performance of fuel cell.

Example 11—Ion-Conducting Cellulose Hydrogel

After filling residual wood channels with polyacrylamide (PAM) hydrogelprecursor by a solution-based method, wood hydrogels were synthesized byfree-radical polymerization. Specifically, acrylamide (AM) monomer,ammonium persulfate (APS) initiator, and N,N′-methylenebisacrylamide(MBA) cross-linker reacted at 60° C. in the delignified wood channelsand formed strong hydrogen bonds with cellulose nanofibers, whichenables outstanding mechanical strength and flexibility. Therefore, thestrengthened skeletons of aligned cellulose nanofibrils, energydissipation of the PAM hydrogel, and strong interfacial bonding of thehydrogel can provide an anisotropic, strong wood-hydrogel composite. Thetensile strength of the wood hydrogel synthesized by this method as 36MPa with an elastic modulus of 310 MPa along the growth direction (L)and 0.54 MPa with an elastic modulus of 0.135 MPa normal to the growthdirection (R). These values are significantly higher than the unmodifiedPAM hydrogel (0.072 MPa tensile strength and 0.01 MPa elastic modulus).This facile method exploits the advantages of the high-tensile strengthof aligned cellulose nanofibril bundles and can be universally appliedto multiple types of hydrogels without losing their intrinsicflexibility, high water content, etc. The as-prepared wood hydrogel alsodemonstrates unique optical and ion transport properties, including hightransparency, optical anisotropy, and nanofluidic ionic behavior.

CONCLUSION

Any of the features illustrated or described with respect to FIGS.5A-6B, 9, 12-14, the above-described cationic membrane, and Examples1-12 can be combined with any other of FIGS. 5A-6B, 9, 12-14, theabove-described cationic membrane, and Examples 1-11 to provide othersystems and embodiments not otherwise illustrated or specificallydescribed herein.

Any of the features illustrated or described with respect to the methodsof FIGS. 3A-3B, 8, 11, and 16, and Examples 1-11 can be combined withthe methods of any other of FIGS. 3A-3B, 8, 11, and 16, and Examples1-11 to provide other methods and embodiments not otherwise illustratedor specifically described herein.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. We thereforeclaim as our invention all that comes within the scope and spirit ofthese claims.

1. An ion-conducting structure, comprising: a metal-fibril complexformed by one or more elementary nanofibrils, each elementary nanofibrilbeing composed of a (plurality of cellulose molecular chains withfunctional groups, each elementary nanofibril having a plurality ofmetal ions, each metal ion acting as a coordination center between thefunctional groups of adjacent cellulose molecular chains so as to form arespective ion transport channel between the cellulose molecular chains,wherein the metal-fibril complex comprises a plurality of second ions,each second ion being disposed within one of the ion transport channelsso as to be intercalated between the corresponding cellulose molecularchains, and wherein the metal-fibril complex is a solid-state structure.2. The ion-conducting structure of claim 1, wherein the metal-fibrilcomplex further comprises polysaccharide, poly(vinyl chloride) (PVC),poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), poly(ethyleneoxide) (PEO), poly(acrylonitrile) (PAN), poly(ethyl methacrylate)(PEMA), poly(methyl methacrylate) (PMMA), poly(ethylene terephthalate)(PET), polyethylene (PE), poly(ethylene naphthalate) (PEN), polyamide(PA), poly(vinylidene chloride) (PVDC), polylactic acid (PLA), orcombinations thereof.
 3. The ion-conducting structure of claim 1,wherein the plurality of metal ions comprises copper (Cu), zinc (Zn),aluminum (Al), calcium (Ca), iron (Fe), or combinations thereof.
 4. Theion-conducting structure of claim 1, wherein the plurality of secondions comprises lithium (Li+), sodium (Na+), potassium (K+), magnesium(Mg+), protons (H+), or combinations thereof.
 5. The ion-conductingstructure of claim 1, wherein a width of each ion transport channel isabout 1 nm, and a spacing between adjacent ion transport channels withineach elementary nanofibril is less than 2 nm. 6-8. (canceled)
 9. Theion-conducting structure of claim 1, wherein the metal-fibril complexhas a conductivity of at least 10⁻⁴ S/cm.
 10. The ion-conductingstructure of claim 1, wherein a total content of water within themetal-fibril complex is less than or equal to 10 wt %. 11-15. (canceled)16. A battery comprising: first and second electrodes; and a separatormembrane between the first and second electrodes, the separator membranecomprising a solid-state metal-fibril complex, wherein one of the firstand second electrodes operates as a cathode and the other of the firstand second electrodes operates as an anode, the solid-state metal-fibrilcomplex is formed by a plurality of first nanofibrils, each firstnanofibril being composed of a plurality of cellulose molecular chainswith first functional groups, each first nanofibril having a pluralityof first metal ions, each first metal ion acting as a first coordinationcenter between the first functional groups of adjacent cellulosemolecular chains so as to form a respective first ion transport channelthrough the separator membrane, and the solid-state metal-fibril complexcomprises a plurality of second ions, each second ion being disposedwithin one of the first ion transport channels so as to be intercalatedbetween the corresponding cellulose molecular chains.
 17. The battery ofclaim 16, wherein: the first electrode, the second electrode, or boththe first and second electrodes comprise a base material and an additiveinterspersed within the base material, the additive comprises one ormore second nanofibrils, each second nanofibril being composed of aplurality of second cellulose molecular chains with second functionalgroups, each second nanofibril having a plurality of second metal ions,each second metal ion acting as a second coordination center between thesecond functional groups of adjacent second cellulose molecular chainsso as to form a respective second ion transport channel between thesecond cellulose molecular chains, and each second nanofibril comprisinga plurality of third ions, each third ion being disposed within arespective one of the second ion transport channels so as to beintercalated between the corresponding second cellulose molecularchains.
 18. A battery comprising: first and second electrodes, one ofthe first and second electrodes operating as a cathode and the other ofthe first and second electrodes operating as an anode; and a separatorbetween the first and second electrodes, the separator comprising asolid-state electrolyte, the first electrode, the second electrode, orboth the first and second electrodes comprise a solid-state metal-fibrilcomplex, the solid-state metal-fibril complex is formed by a pluralityof nanofibrils, each nanofibril being composed of a plurality ofcellulose molecular chains with functional groups, each nanofibrilhaving a plurality of metal ions, each metal ion acting as acoordination center between the functional groups of adjacent cellulosemolecular chains so as to form a respective ion transport channelbetween the cellulose molecular chains, and the solid-state metal-fibrilcomplex comprises a plurality of second ions, each second ion beingdisposed within one of the ion transport channels so as to beintercalated between the corresponding cellulose molecular chains. 19.The battery of claim 18, wherein the solid-state electrolyte comprisesan oxide-based electrolyte, a sulfide-based electrolytes, a polymerelectrolyte, or combinations thereof.
 20. The battery of claim 16,wherein the solid-state metal-fibril complex, the additive, or both thesolid-state metal-fibril complex and the additive further comprisepolysaccharide, poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA),poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), poly(ethyl methacrylate) (PEMA), poly(methylmethacrylate) (PMMA), poly(ethylene terephthalate) (PET), polyethylene(PE), poly(ethylene naphthalate) (PEN), polyamide (PA), poly(vinylidenechloride) (PVDC), polylactic acid (PLA), or combinations thereof. 21.The battery of claim 16, wherein: each metal ion comprises copper (Cu),zinc (Zn), aluminum (Al), calcium (Ca), iron (Fe), or combinationsthereof; and/or the second ions, the third ions, or both the second ionsand the third ions comprise lithium (Li), sodium (Na), potassium (K),magnesium (Mg), or combinations thereof.
 22. The battery of claim 16,wherein: the first electrode operates as the anode and comprisesgraphite, silicon, carbon, or combinations thereof; and/or the secondelectrode operates as the cathode and comprises lithium cobalt oxide(LCO) (LiCoO₂), lithium manganese oxide (LMO) (LiMn₂O₄), lithium ironphosphate (LFP) (LiFePO₄/C), lithium nickel cobalt manganese oxide (NMC)(LiNiCoMnO₂), lithium nickel manganese spinel (LNMO)(LiNi_(0.5)Mn_(1.5)O₄), lithium nickel cobalt aluminum oxide (NCA)(LiNiCoAlO₂), sulfur-carbon (S/C) composite, or combinations thereof.23. The battery of claim 16, wherein: each of the first and secondelectrodes is in contact with the separator membrane; and the separatormembrane is constructed to operate as a solid-state electrolyte betweenthe first and second electrodes.
 24. (canceled)
 25. A method,comprising: (a) forming a metal-fibril complex by immersing a pluralityof elementary nanofibrils within an alkaline solution having aconcentration of at least 5% (w/v) and a plurality of metal ionsdissolved therein, each elementary nanofibril being composed of aplurality of cellulose molecular chains with functional groups, theimmersing being such that hydrogen bonds between adjacent functionalgroups of the cellulose molecular chains are broken so as to expose thefunctional groups and such that the dissolved metal ions from thealkaline solution form coordination bonds with the exposed functionalgroups; (b) after (a), intercalating second ions between adjacentcellulose molecular chains of the metal-fibril complex by immersing themetal-fibril complex in a first solution having a plurality of thesecond ions dissolved therein; (c) after (a), replacing free water inthe metal-fibril complex by immersing the metal-fibril complex in anorganic solvent; and (d) after (c), drying the metal-fibril complex suchthat a total content of water within the metal-fibril complex is lessthan or equal to 10 wt %, thereby forming the metal-fibril complex withintercalated second ions as a solid-state ion conducting structure,wherein: the first solution is the organic solvent, and theintercalating of (b) and the replacing free water of (c) are performedsimultaneously, or the first solution is separate from the organicsolvent, and the intercalating of (b) is performed before or after thereplacing free water of (c).
 26. The method of claim 25, wherein theelementary nanofibrils further comprise polysaccharide, poly(vinylchloride) (PVC), poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA),poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(ethylmethacrylate) (PEMA), poly(methyl methacrylate) (PMMA), poly(ethyleneterephthalate) (PET), polyethylene (PE), poly(ethylene naphthalate)(PEN), polyamide (PA), poly(vinylidene chloride) (PVDC), polylactic acid(PLA), or combinations thereof.
 27. The method of claim 25, wherein theplurality of metal ions comprises copper (Cu), zinc (Zn), aluminum (Al),calcium (Ca), iron (Fe), or combinations thereof.
 28. The method ofclaim 25, wherein the plurality of second ions comprises lithium (Li),sodium (Na), potassium (K), magnesium (Mg), protons (H+), orcombinations thereof.
 29. The method of claim 25, wherein the alkalinesolution comprises sodium hydroxide (NaOH), potassium hydroxide (KOH),lithium hydroxide (LiOH), or combinations thereof.
 30. The method ofclaim 25, wherein the first solution comprises propylene carbonate (PC),ethylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate(EMC), diethyl carbonate (DEC), or combinations thereof.
 31. The methodof claim 25, wherein the organic solvent comprises dimethylformamide(DMF), dimethyl sulfoxide (DMSO), propylene carbonate (PC), acetone,ethylene glycol diglycidyl ether (EGDGE), or combinations thereof.32-34. (canceled)
 35. The method of claim 25, wherein, after (a), iontransport channels are formed between adjacent cellulose molecularchains by the metal ions acting as coordination centers between thefunctional groups, a width of each ion transport channel is about 1 nm,and a spacing between adjacent ion transport channels is less than 2 nm.36. (canceled)
 37. The method of claim 25, further comprising, prior to(a), subjecting a parent structure containing the elementary nanofibrilsto a mechanical fibrillation process, a chemical fibrillation process,an enzymatic fibrillation process, or combinations thereof, so as toexpose the nanofibrils from the parent structure, wherein the parentstructure comprises a block of natural wood or a piece of paper formedof cellulose fibers. 38-92. (canceled)
 93. The battery of claim 18,wherein the solid-state metal-fibril complex, the additive, or both thesolid-state metal-fibril complex and the additive further comprisepolysaccharide, poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA),poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), poly(ethyl methacrylate) (PEMA), poly(methylmethacrylate) (PMMA), poly(ethylene terephthalate) (PET), polyethylene(PE), poly(ethylene naphthalate) (PEN), polyamide (PA), poly(vinylidenechloride) (PVDC), polylactic acid (PLA), or combinations thereof. 94.The battery of claim 18, wherein: each metal ion comprises copper (Cu),zinc (Zn), aluminum (Al), calcium (Ca), iron (Fe), or combinationsthereof; and/or the second ions, the third ions, or both the second ionsand the third ions comprise lithium (Li), sodium (Na), potassium (K),magnesium (Mg), or combinations thereof.
 95. The battery of claim 18,wherein: the first electrode operates as the anode and comprisesgraphite, silicon, carbon, or combinations thereof; and/or the secondelectrode operates as the cathode and comprises lithium cobalt oxide(LCO) (LiCoO₂), lithium manganese oxide (LMO) (LiMn₂O₄), lithium ironphosphate (LFP) (LiFePO₄/C), lithium nickel cobalt manganese oxide (NMC)(LiNiCoMnO₂), lithium nickel manganese spinel (LNMO)(LiNi_(0.5)Mn_(1.5)O₄), lithium nickel cobalt aluminum oxide (NCA)(LiNiCoAlO₂), sulfur-carbon (S/C) composite, or combinations thereof.